Remove -u root as mysql is executed with root user

As openstack installation guides suggest to run mysql with root shell
user, mysql will not ask for password, so the "-u root -p" is useless.

Change-Id: I5ffa77971ecbcc9210b185a39842140b3acd8147
Related-Bug: #1785025

doc/source/install/common/prerequisites.rst

@@ -11,7 +11,7 @@ service, you must create a database, service credentials, and API endpoints.
 
      .. code-block:: console
 
-        $ mysql -u root -p
+        # mysql
 
    * Create the ``magnum`` database: