Merge pull request #224 from clstb/master

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

provider names in issuer.yml and certificate.yml must match

**What this PR does / why we need it**:

Documentation fix. For further information see #209.

**Which issue this PR fixes** 

fixes #209

```release-note
NONE
```

docs/user-guides/acme-dns-validation.md

@@ -64,11 +64,11 @@ spec:
   acme:
     config:
     - dns01:
-        provider: clouddns
+        provider: prod-dns
       domains:
       - example.com
     - dns01:
-        provider: cloudflare
+        provider: cf-dns
       domains:
       - www.example.com
 ```
@@ -109,4 +109,4 @@ Once our certificate has been obtained, cert-manager will periodically check its
   [5]: ../api-types/issuer/spec.md#user-content-acme-issuer-dns-provider-configuration
   [6]: https://en.wikipedia.org/wiki/Subject_Alternative_Name
   [7]: ../api-types/certificate/
-  [8]: cluster-issuers.md
+  [8]: cluster-issuers.md