Fix Python test to use lowercase command

While testing this series I saw some errors from the Python test
suite.  There were a couple of tests using "P" as a command; this
changes them to "p".

gdb/testsuite/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* gdb.python/py-xmethods.exp: Use "p" command, not "P".