[binutils-gdb] Fix switch_back_to_stepped_thread comment references

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT 44a1ee517365cd120f0338c1f4dc5a85273d4e4b ***

Author: Pedro Alves <palves@redhat.com>
Branch: master
Commit: 44a1ee517365cd120f0338c1f4dc5a85273d4e4b

Fix switch_back_to_stepped_thread comment references
Whoops, switch_back_to_stepping doesn't exist...

gdb/
2015-03-24  Pedro Alves  <palves@redhat.com>

	* infrun.c (resume, proceed): Mention
	switch_back_to_stepped_thread, not switch_back_to_stepping.