Updated MySQL documentation link to MySQL latest version 5.7 everywhere [ci skip]

Bumps from `5.6` to `5.7`

1 files changed, 1 insertions, 1 deletions

diff --git a/guides/source/active_record_validations.md b/guides/source/active_record_validations.md
index b99113ed3e..15f65a6501 100644
--- a/guides/source/active_record_validations.md
+++ b/guides/source/active_record_validations.md
@@ -636,7 +636,7 @@ class Holiday < ActiveRecord::Base
     message: "should happen once per year" }
 end
 ```
-Should you wish to create a database constraint to prevent possible violations of a uniqueness validation using the `:scope` option, you must create a unique index on both columns in your database. See [the MySQL manual](http://dev.mysql.com/doc/refman/5.6/en/multiple-column-indexes.html) for more details about multiple column indexes or [the PostgreSQL manual](http://www.postgresql.org/docs/current/static/ddl-constraints.html) for examples of unique constraints that refer to a group of columns.
+Should you wish to create a database constraint to prevent possible violations of a uniqueness validation using the `:scope` option, you must create a unique index on both columns in your database. See [the MySQL manual](http://dev.mysql.com/doc/refman/5.7/en/multiple-column-indexes.html) for more details about multiple column indexes or [the PostgreSQL manual](http://www.postgresql.org/docs/current/static/ddl-constraints.html) for examples of unique constraints that refer to a group of columns.
 
 There is also a `:case_sensitive` option that you can use to define whether the
 uniqueness constraint will be case sensitive or not. This option defaults to