a68: fix %%< and %%> marks in snprintf calls

The function a68_mode_error_text computes a string that is then passed
to a68_error or a68_warning.  The later functions feed the resulting
string to the diagnostics machinery, which knows how to handle %< and
%>, but the *printf calls don't.  Therefore the %s have to be escaped
for %< and %> to be interpreted literally.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

	* a68-moids-diagnostics.cc (a68_mode_error_text): Properly escape
	%< and %> in snprintf calls.