Corrected error message as suggested by @parkr.

Signed-off-by: Martin Jorn Rogalla <martin@martinrogalla.com>

lib/jekyll/filters.rb

@@ -223,7 +223,7 @@ module Jekyll
     # Returns the filtered array of objects
     def sort(input, property = nil, nils = "first")
       if input.nil?
-          raise ArgumentError.new("Sort: cannot sort a null object.")
+          raise ArgumentError.new("Cannot sort a null object.")
       end
       if property.nil?
         input.sort

test/test_filters.rb

@@ -284,7 +284,7 @@ class TestFilters < JekyllUnitTest
         err = assert_raises ArgumentError do
           @filter.sort(nil)
         end
-        assert_equal "Sort: cannot sort a null object.", err.message
+        assert_equal "Cannot sort a null object.", err.message
       end
       should "return sorted numbers" do
         assert_equal [1, 2, 2.2, 3], @filter.sort([3, 2.2, 2, 1])