Update mysql-on-fly.html.markerb to use at least mysql:8.0.37 (#1917)

as reported also elsewhere (e.g. https://community.fly.io/t/error-deploying-mysql-database/20008/4 ) the mysql:9 image suggested in the exmple fails do work in the end (mysqld keeps erroring out due to missing mysql.component and default auth plugin).

This is quickjly fixed by using at least v 8.0.37. haven't checked other versions

Author: mabis

app-guides/mysql-on-fly.html.markerb

@@ -29,7 +29,7 @@ cd my-mysql
 # Run `fly launch` to create an app
 # Use the --no-deploy option since we'll make changes before first deploy
 # Use the --image option to specify a MySQL Docker image
-fly launch --no-deploy --image mysql:8
+fly launch --no-deploy --image mysql:8.0.37
 ```
 
 Type `y` when prompted to tweak the default app settings. Then, on the Fly Launch page: