README.md: <Leader> is actually <LocalLeader>

Fix documentation <=> code discrepancy.

[1] explains the reasoning behind using <LocalLeader> over <Leader>:

  The <Leader> prefix should be used for global mappings (applicable to
  all buffers) defined by a plugin. For buffer-local mappings, the
  <LocalLeader> prefix should be used. Vim will replace this with the
  value set in the 'maplocalleader' variable. The default value for this
  variable is backslash ('\'). The <LocalLeader> is generally used in
  mappings defined by a Vim filetype plugin.

  The 'mapleader' and 'maplocalleader' variables allow the user to choose
  different keys as starting keys for global mappings and buffer-local
  mappings defined by Vim plugins.

[1]: http://vim.wikia.com/wiki/Mapping_keys_in_Vim_-_Tutorial_%28Part_3%29
README.md: fix typo s/Graphiz/Graphviz/
Merge pull request #9 from bjornfor/doc-fixes

Doc fixes