Fix typos (#610)

lib/ecto/adapters/tds/types.ex

@@ -1,7 +1,7 @@
 if Code.ensure_loaded?(Tds) do
   defmodule Tds.Ecto.UUID do
     @moduledoc """
-    An TDS adapter type for UUIDs strings.
+    A TDS adapter type for UUIDs strings.
 
     If you are using Tds adapter and UUIDs in your project, instead of `Ecto.UUID`
     you should use Tds.Ecto.UUID to generate correct bytes that should be stored
@@ -213,7 +213,7 @@ if Code.ensure_loaded?(Tds) do
 
   defmodule Tds.Ecto.VarChar do
     @moduledoc """
-    An Tds adapter Ecto Type that wraps erlang string into tuple so TDS driver
+    A Tds adapter Ecto Type that wraps erlang string into tuple so TDS driver
     can understand if erlang string should be encoded as NVarChar or Varchar.
 
     Due to some limitations in Ecto and Tds driver, it is not possible to

lib/ecto/migration.ex

@@ -1261,7 +1261,7 @@ defmodule Ecto.Migration do
   the corresponding foreign key constraints before modifying the type.
   Generally speaking, you want to pass the type and each option
   you are modifying to `:from`, so the column can be rolled back properly.
-  However, note that `:from` cannot be be used to modify primary keys,
+  However, note that `:from` cannot be used to modify primary keys,
   as those are generally trickier to revert.
 
   See `add/3` for more information on supported types.