[ci skip] Update MySQL docs reference to latest

Bumps to 5.7

guides/source/active_record_querying.md

@@ -1865,6 +1865,6 @@ following pointers may be helpful:
 
 * SQLite3: [EXPLAIN QUERY PLAN](http://www.sqlite.org/eqp.html)
 
-* MySQL: [EXPLAIN Output Format](http://dev.mysql.com/doc/refman/5.6/en/explain-output.html)
+* MySQL: [EXPLAIN Output Format](http://dev.mysql.com/doc/refman/5.7/en/explain-output.html)
 
 * PostgreSQL: [Using EXPLAIN](http://www.postgresql.org/docs/current/static/using-explain.html)