Server example wrongly applies healthcheck layer in B position

[david-perez]

Our Pokemon service example applies AlbHealthCheckLayer in B position after routing. This is a bad example because health checks should run before routing, since typically they should not be modelled.

Docs for the aws_smithy_http_server::layer module explicitly mention that layers within it should wrap the router; it wouldn't hurt to link to B position middleware in the Rust book.

This appears to be a regression. When we created the module in #2865 the example worked as intended but when we introduced the generated config object we messed it up.