Fix typo

Introduced in 90fdc259866e; backpatch to 12.

Author: Erik Rijkers <er@xs4all.nl>
Discussion: https://postgr.es/m/e92b3fba98a0c0f7afc0a2a37e765954@xs4all.nl

doc/src/sgml/ref/create_index.sgml

@@ -746,7 +746,7 @@ Indexes:
   </para>
 
   <para>
-   The regularly system collects statistics on all of a table's
+   The system regularly collects statistics on all of a table's
    columns.  Newly-created non-expression indexes can immediately
    use these statistics to determine an index's usefulness.
    For new expression indexes, it is necessary to run <link