[docs] correcting code-block rst syntax

When building the html documentation with `make html` in the `docs`
directory one receives the following error message:

    behave/docs/parse_builtin_types.rst:10: ERROR: Unknown directive type "code".

The directive should be `code-block`: this commit corrects this syntax
error.
[docs] removed duplicated word: 'with'
[docs] using plural for 'Background'

In the context of the sentence containing this change, it seems that
'Backgrounds' was meant instead of merely 'Background', since 'Scenarios' is
also used in the same sentence and not 'Scenario' which would have been the
relevant keyword.
[docs] corrected 'stricltly' typo
[docs] padding out tag lines for html to match rst

When showing examples of valid tag lines the list shows up in a single line
in the rendered html, and since the @slow keyword turns up twice this does
look odd.  The rst, however, shows these tag lines on separate lines and it
therefore seems that the desired html formatting was actually three separate
tag lines.  This change makes the rendered html match the rst's intention.
Additional documentation tweaks.