wrap match name in quote to make sure it work

This maybe problem in code but I just fix document since that worked.

I got throwed in to this Exception
https://github.com/saltstack/salt/blob/develop/salt/overstate.py#L45
and had no idea why. 
It's better to see the error than catch Exception.

I tried to debug the code, then figured out that because I used match: *word
```yaml
master:
  match: *master
  sls:
    - postgresql.server.master

standby:
  match: *standby
  sls:
    - postgresql.server.standby
  require:
    - master
```
After wrap *master => '*master' , same for *standby, it worked