Replaced regular apostrophes by "typewriter apostrophes"

This is again a problem caused by writing the original document
using Microsoft Word. The ortographic apostrophes (‘ and ’) aren't
used in technical documents, because you cannot easily write them
with the IDEs and text editors used by developers.

The solution is to use the "typewriter apostrophe" (') in both cases.
Merge pull request #6

* pull-request/6:
  Replaced regular apostrophes by "typewriter apostrophes"

Conflicts:
	spec/php-spec-draft.md