Minor fix to gdb.prompt documentation

Minor fix to gdb.prompt documentation

I noticed that an example in the gdb.prompt documentation used the
wrong kind of quotes -- because it is code, it should use a plain
ASCII quotation mark.  I also slightly shortened the sample text here,
so it would more clearly fit on a single line.

gdb/doc/ChangeLog
2019-12-10  Tom Tromey  <tom@tromey.com>

	* python.texi (gdb.prompt): Use correct quotes in example.
	Shorten sample text.

Change-Id: I4153928c0d88001244ad410f3943c952a6ebfeb1