Update version in upgrade.md

Author: deliahu

docs/cluster/upgrade.md

@@ -13,7 +13,7 @@ See [cluster configuration](config.md) to customize your installation.
 
 ```bash
 # Download
-curl -O https://raw.githubusercontent.com/cortexlabs/cortex/master/cortex.sh
+curl -O https://raw.githubusercontent.com/cortexlabs/cortex/0.7/cortex.sh
 
 # Change permissions
 chmod +x cortex.sh