Merge pull request #1142 from clarksun/patch-1

exception.md code sample miss 'async' prefix

docs/sanic/exceptions.md

@@ -13,7 +13,7 @@ To throw an exception, simply `raise` the relevant exception from the
 from sanic.exceptions import ServerError
 
 @app.route('/killme')
-def i_am_ready_to_die(request):
+async def i_am_ready_to_die(request):
 	raise ServerError("Something bad happened", status_code=500)
 ```
 
@@ -24,7 +24,7 @@ from sanic.exceptions import abort
 from sanic.response import text
 
 @app.route('/youshallnotpass')
-def no_no(request):
+async def no_no(request):
         abort(401)
         # this won't happen
         text("OK")
@@ -43,7 +43,7 @@ from sanic.response import text
 from sanic.exceptions import NotFound
 
 @app.exception(NotFound)
-def ignore_404s(request, exception):
+async def ignore_404s(request, exception):
 	return text("Yep, I totally found the page: {}".format(request.url))
 ```