[ci skip] fix doc typo for validates_uniqueness_of

Author: shizm

activerecord/lib/active_record/validations/uniqueness.rb

@@ -152,7 +152,7 @@ module ClassMethods
       #   or <tt>if: Proc.new { |user| user.signup_step > 2 }</tt>). The method,
       #   proc or string should return or evaluate to a +true+ or +false+ value.
       # * <tt>:unless</tt> - Specifies a method, proc or string to call to
-      #   determine if the validation should ot occur (e.g. <tt>unless: :skip_validation</tt>,
+      #   determine if the validation should not occur (e.g. <tt>unless: :skip_validation</tt>,
       #   or <tt>unless: Proc.new { |user| user.signup_step <= 2 }</tt>). The
       #   method, proc or string should return or evaluate to a +true+ or +false+
       #   value.