Fix various typos and spelling mistakes in code comments

Author: Justin Pryzby
Discussion: https://postgr.es/m/20220411020336.GB26620@telsasoft.com

src/backend/commands/dbcommands.c

@@ -386,7 +386,7 @@ ScanSourceDatabasePgClassPage(Page page, Buffer buf, Oid tbid, Oid dbid,
  * needs to be copied from the source database to the destination database,
  * and if so, construct a CreateDBRelInfo for it.
  *
- * Visbility checks are handled by the caller, so our job here is just
+ * Visibility checks are handled by the caller, so our job here is just
  * to assess the data stored in the tuple.
  */
 CreateDBRelInfo *

src/backend/commands/vacuumparallel.c

@@ -12,7 +12,7 @@
  * the memory space for storing dead items allocated in the DSM segment.  We
  * launch parallel worker processes at the start of parallel index
  * bulk-deletion and index cleanup and once all indexes are processed, the
- * parallel worker processes exit.  Each time we process indexes parallelly,
+ * parallel worker processes exit.  Each time we process indexes in parallel,
  * the parallel context is re-initialized so that the same DSM can be used for
  * multiple passes of index bulk-deletion and index cleanup.
  *