Version Control With Subversion Source

Patch from Chris Pepper <pepper@...>, tweaked.

* doc/book/misc-docs/directory_versioning.xml
Singular vs. plural fix.
Strike out-of-place 'a '.
Strike extra space before </para>.

* doc/book/misc-docs/best_practices.xml
"Mac OS" contains a space.
Out-of-place period.
s/can not/cannot/.

* doc/book/misc-docs/quick_walkthrough.xml
Out-of-place 'the'.