Update extensions.md "book(s)" to "guide(s)"

Changed "book(s)" to "guide(s)" throughout.

NOTE: No change: the two following lines have links with "book(s)" embedded in them.
Line no.	issue
12		[/**books**/volunteer.yml](https://github.com/civicrm/civicrm-docs/blob/master/**books**/volunteer.yml)
101		Go to the [**books** configuration within our publishing system](https://github.com/civicrm/civicrm-docs/tree/master/**books**)