Update gelf.md (#1259)

Missed this link on my last  update. Fixing GELF Specification list.

Signed-off-by: Joshua Cooper <[email protected]>

pipeline/outputs/gelf.md

@@ -103,7 +103,7 @@ Now, this is what happens to this log:
 3. We used this `data` key as `Gelf_Short_Message_Key`; so GELF plugin changes it to `short_message`.
 4. [Kubernetes Filter](../filters/kubernetes.md) adds `host` name.
 5. Timestamp is generated.
-6. Any custom field (not present in [GELF Payload Specification](https://docs.graylog.org/en/latest/pages/gelf.html#gelf-payload-specification)) is prefixed by an underline.
+6. Any custom field (not present in [GELF Payload Specification](https://go2docs.graylog.org/5-0/getting_in_log_data/gelf.html?Highlight=Payload#GELFPayloadSpecification).) is prefixed by an underline.
 
 Finally, this is what our Graylog server input sees: