Fix typos in comments.

Author: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoA1YL7t0nzVSEySx6zOaE7xO3r0jyu8hkitGL2_XbaMxQ@mail.gmail.com

contrib/amcheck/verify_heapam.c

@@ -777,7 +777,7 @@ check_tuple_visibility(HeapCheckContext *ctx)
 				case XID_COMMITTED:
 					/*
 					 * The tuple is dead, because the xvac transaction moved
-					 * it off and comitted. It's checkable, but also prunable.
+					 * it off and committed. It's checkable, but also prunable.
 					 */
 					return true;
 
@@ -851,7 +851,7 @@ check_tuple_visibility(HeapCheckContext *ctx)
 				case XID_ABORTED:
 					/*
 					 * The tuple is dead, because the xvac transaction moved
-					 * it off and comitted. It's checkable, but also prunable.
+					 * it off and committed. It's checkable, but also prunable.
 					 */
 					return true;
 			}