fix(pillar.example): fix yamllint error

* https://travis-ci.org/myii/bind-formula/builds/594704904#L211-L213

```bash
$ yamllint -s .
./pillar.example
  26:29     error    forbidden implicit octal value "008"  (octal-values)
```

Author: myii

pillar.example

@@ -23,7 +23,7 @@ bind:
   lookup:
     key_directory: '/etc/bind/keys'               # Key directory (needed to use auto-dnssec)
     key_algorithm: RSASHA256                      # Algorithm when using auto-dnssec
-    key_algorithm_field: 008                      # See http://www.bind9.net/dns-sec-algorithm-numbers
+    key_algorithm_field: '008'                    # See http://www.bind9.net/dns-sec-algorithm-numbers
     key_size: 4096                                # Key size
 
   config: