Pluralize "index" (#18811)

This doesn't just happen to "an index" unless you're restoring just one.  It reads better this way, IMO.

docs/reference/modules/snapshots.asciidoc

@@ -328,7 +328,7 @@ By default, all indices in the snapshot are restored, and the cluster state is
 as to allow the global cluster state from being restored by using `indices` and
 `include_global_state` options in the restore request body. The list of indices
 supports <<search-multi-index-type,multi index syntax>>. The `rename_pattern`
-and `rename_replacement` options can be also used to rename index on restore
+and `rename_replacement` options can be also used to rename indices on restore
 using regular expression that supports referencing the original text as
 explained
 http://docs.oracle.com/javase/6/docs/api/java/util/regex/Matcher.html#appendReplacement(java.lang.StringBuffer,%20java.lang.String)[here].