Fix wrong articles in pg_proc descriptions.

This technically should involve a catversion bump, but that seems
pedantic, so I skipped it.

Report and patch by David Christensen.

src/include/catalog/pg_proc.h

@@ -3160,9 +3160,9 @@ DESCR("current wal insert location");
 DATA(insert OID = 3330 ( pg_current_wal_flush_location PGNSP PGUID 12 1 0 0 0 f f f f t f v s 0 0 3220 "" _null_ _null_ _null_ _null_ _null_ pg_current_wal_flush_location _null_ _null_ _null_ ));
 DESCR("current wal flush location");
 DATA(insert OID = 2850 ( pg_walfile_name_offset PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 2249 "3220" "{3220,25,23}" "{i,o,o}" "{wal_location,file_name,file_offset}" _null_ _null_ pg_walfile_name_offset _null_ _null_ _null_ ));
-DESCR("wal filename and byte offset, given an wal location");
+DESCR("wal filename and byte offset, given a wal location");
 DATA(insert OID = 2851 ( pg_walfile_name			PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 25 "3220" _null_ _null_ _null_ _null_ _null_ pg_walfile_name _null_ _null_ _null_ ));
-DESCR("wal filename, given an wal location");
+DESCR("wal filename, given a wal location");
 
 DATA(insert OID = 3165 ( pg_wal_location_diff		PGNSP PGUID 12 1 0 0 0 f f f f t f i s 2 0 1700 "3220 3220" _null_ _null_ _null_ _null_ _null_ pg_wal_location_diff _null_ _null_ _null_ ));
 DESCR("difference in bytes, given two wal locations");