gdb/doc: reword a sentence

Change this:

  The available watchpoint types represented by constants are defined
  in the gdb module:

to this:

  The available watchpoint types are represented by constants defined
  in the gdb module:

The new version matches a similar line a few lines up the document
which reads:

  The available types are represented by constants defined in the gdb
  module:

gdb/doc/ChangeLog:

	* guile.texinfo (Breakpoints In Guile): Reword sentence.
	* python.texinfo (Breakpoints In Python): Reword sentence.