README example uses wrong property names for access token properties

[benjisg]

The current README shows that you can access the property names of an access token object by using:

// Use this to interact with an API on the users behalf
echo $token->access_token;

// Use this to get a new access token if the old one expires
echo $token->refresh_token;

// Number of seconds until the access token will expire, and need refreshing
echo $token->expires_in;

but the actual property names would need to be

$token->accessToken;
$token->refreshToken;
$token->expires;