Update obsolete reference to plpgsql's gram.y file.

This was evidently missed in 05346c131, which renamed that
file to pl_gram.y.

Japin Li

Discussion: https://postgr.es/m/ME0P300MB0445F7CA7456C2AC67D37A01B6092@ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

src/pl/plpgsql/src/plpgsql.h

@@ -1333,7 +1333,7 @@ extern void plpgsql_scanner_init(const char *str);
 extern void plpgsql_scanner_finish(void);
 
 /*
- * Externs in gram.y
+ * Externs in pl_gram.y
  */
 extern int	plpgsql_yyparse(void);