Fix garbled mirror configuration in recipe for git clone --mirror

[cjolowicz]

The documentation contains a recipe for how to emulate git clone --mirror. In
the example code, the init_remote callback constructs the mirror configuration
passing name to str.format. However, the callback receives the name (and
url) as bytes, so str.format would return something like this:

"remote.b'origin'.mirror"

This commit fixes the code example by decoding name when used with str.format.

The invocation of repo.remotes.create in the same example works fine as is,
because that function invokes to_bytes on its arguments, and to_bytes is a
noop when passed bytes.