Make @pxref for Inferior.architecture point to gdb.Frame documentation

Make @pxref for Inferior.architecture point to gdb.Frame documentation

This fixes he @pxref in Inferior.architecture to point to the "Frames
In Python" node, as originally intended; somewhat reverting an earlier
build fix.  The initial patch had typod the "In".

Tested by "make info".

gdb/doc/ChangeLog
2018-10-09  Tom Tromey  <tom@tromey.com>

	* python.texi (Inferiors In Python): Link to "Frames In Python",
	not "Unwinding Frames in Python".