Ensure we use the correct spelling of "ensure"

We seem to have accidentally used "insure" in a few places.  Correct
that.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Pv0biqrhA3pMhu40aDsj343mTsD75khKnHsLqR8P04f=Q@mail.gmail.com
Backpatch-through: 12, oldest supported version

src/backend/access/heap/hio.c

@@ -387,7 +387,7 @@ RelationGetBufferForTuple(Relation relation, Size len,
 	 * on, as cached in the BulkInsertState or relcache entry.  If that
 	 * doesn't work, we ask the Free Space Map to locate a suitable page.
 	 * Since the FSM's info might be out of date, we have to be prepared to
-	 * loop around and retry multiple times. (To insure this isn't an infinite
+	 * loop around and retry multiple times. (To ensure this isn't an infinite
 	 * loop, we must update the FSM with the correct amount of free space on
 	 * each page that proves not to be suitable.)  If the FSM has no record of
 	 * a page with enough free space, we give up and extend the relation.

src/backend/utils/misc/guc.c

@@ -1226,7 +1226,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"fsync", PGC_SIGHUP, WAL_SETTINGS,
 			gettext_noop("Forces synchronization of updates to disk."),
 			gettext_noop("The server will use the fsync() system call in several places to make "
-						 "sure that updates are physically written to disk. This insures "
+						 "sure that updates are physically written to disk. This ensures "
 						 "that a database cluster will recover to a consistent state after "
 						 "an operating system or hardware crash.")
 		},

src/include/storage/buf_internals.h

@@ -150,7 +150,7 @@ typedef struct buftag
  * is held.  Thus buffer header lock holder can do complex updates of the
  * state variable in single write, simultaneously with lock release (cleaning
  * BM_LOCKED flag).  On the other hand, updating of state without holding
- * buffer header lock is restricted to CAS, which insure that BM_LOCKED flag
+ * buffer header lock is restricted to CAS, which ensures that BM_LOCKED flag
  * is not set.  Atomic increment/decrement, OR/AND etc. are not allowed.
  *
  * An exception is that if we have the buffer pinned, its tag can't change