docs: Fix a copy-paste error on get_username docs (#3786)

I suspect this is a copy-paste error, it's meant to say username, not port.

Author: sulami

sqlx-mysql/src/options/mod.rs

@@ -448,7 +448,7 @@ impl MySqlConnectOptions {
         self.socket.as_ref()
     }
 
-    /// Get the server's port.
+    /// Get the current username.
     ///
     /// # Example
     ///