Update README.md

For Ubuntu setup instructions, adding call to `sudo apt-get update` before running `sudo apt-get install azure-functions-core-tools`. Otherwise, the listing of the newly-added repository is not fetched and the following message occurs upon install attempt:

```
E: Unable to locate package azure-functions-core-tools
```

Author: yevster

README.md

@@ -83,6 +83,7 @@ sudo dpkg -i packages-microsoft-prod.deb
 2. Install
 
 ```bash
+sudo apt-get update
 sudo apt-get install azure-functions-core-tools
 ```