make guides example more friendly to nonlatin langs

guides/source/active_record_validations.md

@@ -706,7 +706,7 @@ we don't want names and surnames to begin with lower case.
 ```ruby
 class Person < ActiveRecord::Base
   validates_each :name, :surname do |record, attr, value|
-    record.errors.add(attr, 'must start with upper case') if value =~ /\A[a-z]/
+    record.errors.add(attr, 'must start with upper case') if value =~ /\A[[:alpha:]]/
   end
 end
 ```