Fix typo in comment

Introduced by 8b08f7d4; backpatch to 11.

Discussion: https://postgr.es/m/20200812214918.GA30353@alvherre.pgsql

src/bin/pg_dump/pg_dump.h

@@ -368,7 +368,7 @@ typedef struct _indxInfo
 								 * contains both key and nonkey attributes */
 	bool		indisclustered;
 	bool		indisreplident;
-	Oid			parentidx;		/* if partitioned, parent index OID */
+	Oid			parentidx;		/* if a partition, parent index OID */
 	SimplePtrList partattaches; /* if partitioned, partition attach objects */
 
 	/* if there is an associated constraint object, its dumpId: */