Fixes documentation typo.

Documentation had extra colon after keyword.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -761,9 +761,9 @@ def foreign_keys(table_name)
       # [<tt>:name</tt>]
       #   The constraint name. Defaults to <tt>fk_rails_<identifier></tt>.
       # [<tt>:on_delete</tt>]
-      #   Action that happens <tt>ON DELETE</tt>. Valid values are +:nullify+, +:cascade:+ and +:restrict+
+      #   Action that happens <tt>ON DELETE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
       # [<tt>:on_update</tt>]
-      #   Action that happens <tt>ON UPDATE</tt>. Valid values are +:nullify+, +:cascade:+ and +:restrict+
+      #   Action that happens <tt>ON UPDATE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
       def add_foreign_key(from_table, to_table, options = {})
         return unless supports_foreign_keys?