preload! -> preload_app! in docs/deployment.md (#2663)

Signed-off-by: KID the Euforia <kid0725+github@gmail.com>

[ci skip]

docs/deployment.md

@@ -100,7 +100,7 @@ You probably will want to deploy some new code at some point, and you'd like
 puma to start running that new code. Minimizing the amount of time the server
 is unavailable would be nice as well. Here's how to do it:
 
-1. Don't use `preload!`. This dirties the master process and means it will have
+1. Don't use `preload_app!`. This dirties the master process and means it will have
 to shutdown all the workers and re-exec itself to get your new code. It is not compatible with phased-restart and `prune_bundler` as well.
 
 1. Use `prune_bundler`. This makes it so that the cluster master will detach itself