pgsql: Clarify to the translator that plpgsql_yyerror() deals with the

Log Message:
-----------
Clarify to the translator that plpgsql_yyerror() deals with the translation
of "syntax error", not the literal string.

This same change was made in the backend a while ago; but it applies to
plpgsql as well.

Modified Files:
--------------
pgsql/src/pl/plpgsql/src:
scan.l (r1.69 -> r1.70)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l?r1=1.69&r2=1.70)