Replace spurious whitespace

Originally we had an emoji there, but the spellchecker does
not like it. The exclamation mark conveys a bit of that.

Author: fxn

guides/source/classic_to_zeitwerk_howto.md

@@ -154,7 +154,7 @@ Rails.autoloaders.each do |autoloader|
 end
 ```
 
-With that in place, the check passes :
+With that in place, the check passes!
 
 ```
 % bin/rails zeitwerk:check