fortran/openmp.cc: Remove 's' that slipped in during %<..%> replacement

Seemingly, 's' (in VI that's the 's'ubstitute command) appeared verbatim in
a gfc_error message when to doing the '...' to %<...%> replacements in commit
r13-4590-g84f6f8a2a97f88be01e223c9c9dbab801a4f501f

gcc/fortran/
	* openmp.cc (gfc_match_omp_context_selector_specification):
	Remove spurious 's' in an error message.