docs/migration: explicitly set binary mode for open() (#231)

open() uses non-binary by default.  The equivalent pkg_resources.resource_stream() method opens in binary mode.

docs/migration.rst

@@ -91,7 +91,7 @@ bytes.  E.g.::
 The equivalent code in ``importlib_resources`` is pretty straightforward::
 
     ref = importlib_resources.files('my.package').joinpath('resource.dat')
-    with ref.open() as fp:
+    with ref.open('rb') as fp:
         my_bytes = fp.read()