Fix error message on prefix mismatch (#350)

Author: maltoe

lib/ecto/migration.ex

@@ -1297,7 +1297,7 @@ defmodule Ecto.Migration do
         index_or_table
       true ->
         raise Ecto.MigrationError,  message:
-          "the :prefix option `#{prefix}` does match the migrator prefix `#{runner_prefix}`"
+          "the :prefix option `#{prefix}` does not match the migrator prefix `#{runner_prefix}`"
     end
   end
 

test/ecto/migration_test.exs

@@ -550,7 +550,7 @@ defmodule Ecto.MigrationTest do
   @tag prefix: :bar
   test "forward: raise error when prefixes don't match" do
     assert_raise Ecto.MigrationError,
-                 "the :prefix option `foo` does match the migrator prefix `bar`", fn ->
+                 "the :prefix option `foo` does not match the migrator prefix `bar`", fn ->
       create(table(:posts, prefix: "foo"))
       flush()
     end