uses serial / oxford comma (#45990)

Author: timacdonald

src/Illuminate/Translation/lang/en/validation.php

@@ -19,7 +19,7 @@
     'after' => 'The :attribute field must be a date after :date.',
     'after_or_equal' => 'The :attribute field must be a date after or equal to :date.',
     'alpha' => 'The :attribute field must only contain letters.',
-    'alpha_dash' => 'The :attribute field must only contain letters, numbers, dashes and underscores.',
+    'alpha_dash' => 'The :attribute field must only contain letters, numbers, dashes, and underscores.',
     'alpha_num' => 'The :attribute field must only contain letters and numbers.',
     'array' => 'The :attribute field must be an array.',
     'ascii' => 'The :attribute field must only contain single-byte alphanumeric characters and symbols.',