Use consistent Host in mysql config examples

Currently it creates a user @'127.0.0.1' but then tries to grant perms to @'localhost'. 

I don't know which one of these two you would prefer to be in the documentation, but at the moment the instructions do not work.

Author: acottre

documentation/docs/install-pmm/install-pmm-client/connect-database/mysql/mysql.md

@@ -104,7 +104,7 @@ This example creates a pmm user account that has just enough access to collect m
 
     ```sql
     CREATE USER 'pmm'@'127.0.0.1' IDENTIFIED BY '<your_strong_password>' WITH MAX_USER_CONNECTIONS 10;
-    GRANT SELECT, PROCESS, REPLICATION CLIENT, RELOAD ON *.* TO 'pmm'@'localhost';
+    GRANT SELECT, PROCESS, REPLICATION CLIENT, RELOAD ON *.* TO 'pmm'@'127.0.0.1';
     ```
 
 === "On MySQL 8.x"