docs: replace Mysql with MySQL

Author: kenjis

user_guide_src/source/database/query_builder.rst

@@ -953,7 +953,7 @@ $builder->upsertBatch()
 Generates an upsert string based on the data you supply, and runs the
 query. You can either pass an **array** or an **object** to the
 method. By default a constraint will be defined in order. A primary
-key will be selected first and then unique keys. Mysql will use any
+key will be selected first and then unique keys. MySQL will use any
 constraint by default. Here is an example using an array:
 
 .. literalinclude:: query_builder/108.php