README: Improve highstate data capture example.

Improve highstate data capture example in README.md

Also fixes syntax errors that may result from attempting to parse the human output as YAML.  For example:


```sh
$ cat highstate.yaml | python salt-state-graph/salt-state-graph.py > highstate.dot
Traceback (most recent call last):
  File "salt-state-graph/salt-state-graph.py", line 114, in <module>
    main(sys.stdin)
  File "salt-state-graph/salt-state-graph.py", line 75, in main
    state_obj = yaml.safe_load(input)
  File "/usr/local/lib/python2.7/dist-packages/yaml/__init__.py", line 93, in safe_load
    return load(stream, SafeLoader)
  File "/usr/local/lib/python2.7/dist-packages/yaml/__init__.py", line 71, in load
    return loader.get_single_data()
  File "/usr/local/lib/python2.7/dist-packages/yaml/constructor.py", line 37, in get_single_data
    node = self.get_single_node()
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 36, in get_single_node
    document = self.compose_document()
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 55, in compose_document
    node = self.compose_node(None, None)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 82, in compose_node
    node = self.compose_sequence_node(anchor)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 111, in compose_sequence_node
    node.value.append(self.compose_node(node, index))
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 82, in compose_node
    node = self.compose_sequence_node(anchor)
  File "/usr/local/lib/python2.7/dist-packages/yaml/composer.py", line 110, in compose_sequence_node
    while not self.check_event(SequenceEndEvent):
  File "/usr/local/lib/python2.7/dist-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "/usr/local/lib/python2.7/dist-packages/yaml/parser.py", line 379, in parse_block_sequence_first_entry
    return self.parse_block_sequence_entry()
  File "/usr/local/lib/python2.7/dist-packages/yaml/parser.py", line 384, in parse_block_sequence_entry
    if not self.check_token(BlockEntryToken, BlockEndToken):
  File "/usr/local/lib/python2.7/dist-packages/yaml/scanner.py", line 116, in check_token
    self.fetch_more_tokens()
  File "/usr/local/lib/python2.7/dist-packages/yaml/scanner.py", line 257, in fetch_more_tokens
    % ch.encode('utf-8'), self.get_mark())
yaml.scanner.ScannerError: while scanning for the next token
found character '%' that cannot start any token
  in "<stdin>", line 829, column 15

$ cat -n highstate.yaml
<snip>
820  mysudogroup:
821      __env__:
822          base
823      __sls__:
824          common
825      file:
826          - name:
827              /etc/sudoers
828          - text:
829              - %mysudogroup ALL=(ALL) NOPASSWD: ALL
```