Corrects one of the 'up url' commands, was missing -s to specify stage (#692)

Author: desm

docs/07-guides.md

@@ -219,7 +219,7 @@ Open both in the browser:
 
 ```
 $ up url -o
-$ up url production -o
+$ up url -s production -o
 ```
 
 You should see "Hello World from production" and "Hello World from staging".