guile: Fix typos in the manual.

gdb/
2014-02-17  Ludovic Courtès  <ludo@gnu.org>

	* doc/guile.texi (Writing a Guile Pretty-Printer) <example>:
	Remove 'string-begins-with' procedure.  Add 'pretty-printer'
	parameter to 'str-lookup-function' procedure.  Look for
	the "std::string<" prefix.
	(Frames In Guile): Add 'frame' parameter to 'frame-read-var'.
	Adjust description.