Use Str::snake delimiter when formatting attribute names

Simplifies getDisplayableAttribute by passing the space delimiter
directly to Str::snake instead of applying a separate str_replace.

Author: lhsazevedo

src/Illuminate/Validation/Concerns/FormatsMessages.php

@@ -295,7 +295,7 @@ public function getDisplayableAttribute($attribute)
                 : $attribute;
         }
 
-        return str_replace('_', ' ', Str::snake($attribute));
+        return Str::snake($attribute, ' ');
     }
 
     /**