- make blueprint add_route method support view instance

- update documentation that doesn't specify url_prefix parameter

docs/sanic/blueprints.md

@@ -131,8 +131,8 @@ can be used to implement our API versioning scheme.
 from sanic.response import text
 from sanic import Blueprint
 
-blueprint_v1 = Blueprint('v1')
-blueprint_v2 = Blueprint('v2')
+blueprint_v1 = Blueprint('v1', url_prefix='/v1')
+blueprint_v2 = Blueprint('v2', url_prefix='/v2')
 
 @blueprint_v1.route('/')
 async def api_v1_root(request):