Version Control With Subversion Source

Changed occurrences of 'revision control system' to 'version control
system' in the book, as specified in the HACKING file.

Patch from Zack Brown <zbrown@...>

* doc/book/book/ch01.xml: changed 'revision control system' to 'version
control system'

* doc/book/book/ch03.xml: changed 'revision control system' to 'version
control system'

* doc/book/book/ch04.xml: changed 'revision control system' to 'version
control system'