doc: Fix some standard named pattern documentation modes

Currently these use `@var{m3}` but the 3 here is a literal 3
and not part of the mode itself so it should not be inside
the var. Fixed as such.

Built the documentation to make sure it looks correct now.

gcc/ChangeLog:

	* doc/md.texi (widen_ssum, widen_usum, smulhs, umulhs,
	smulhrs, umulhrs, sdiv_pow2): Move the 3 outside of the
	var.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>