Fix recently introduced typos

This fixes typos in docs and comments introduced during the v18
development cycle, to keep them from ending up in backbranches.

Author: Jacob Brazeal <jacob.brazeal@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA+COZaCgGua25f2hSrjrDLJcJJAHkwoKgTTqUy-wyL1=64JNjw@mail.gmail.com

src/backend/statistics/attribute_stats.c

@@ -547,7 +547,7 @@ get_attr_expr(Relation rel, int attnum)
 		return NULL;
 
 	/*
-	 * The index attnum points directly to a relation attnum, then it's no an
+	 * The index attnum points directly to a relation attnum, then it's not an
 	 * expression attribute.
 	 */
 	if (rel->rd_index->indkey.values[attnum - 1] != 0)

src/backend/storage/lmgr/lwlock.c

@@ -1832,7 +1832,7 @@ LWLockDisownInternal(LWLock *lock)
 
 /*
  * Helper function to release lock, shared between LWLockRelease() and
- * LWLockeleaseDisowned().
+ * LWLockReleaseDisowned().
  */
 static void
 LWLockReleaseInternal(LWLock *lock, LWLockMode mode)

src/backend/utils/activity/pgstat_backend.c

@@ -100,7 +100,7 @@ pgstat_fetch_stat_backend(ProcNumber procNumber)
 /*
  * Returns statistics of a backend by pid.
  *
- * This routine includes sanity checks to ensire that the backend exists and
+ * This routine includes sanity checks to ensure that the backend exists and
  * is running.  "bktype" can be optionally defined to return the BackendType
  * of the backend whose statistics are returned.
  */

src/backend/utils/cache/inval.c

@@ -1744,7 +1744,7 @@ CacheInvalidateSmgr(RelFileLocatorBackend rlocator)
 
 	/* verify optimization stated above stays valid */
 	StaticAssertStmt(MAX_BACKENDS_BITS <= 23,
-					 "MAX_BACKEND_BITS is too big for inval.c");
+					 "MAX_BACKENDS_BITS is too big for inval.c");
 
 	msg.sm.id = SHAREDINVALSMGR_ID;
 	msg.sm.backend_hi = rlocator.backend >> 16;