docs(runbooks): add code-fence languages to satisfy markdownlint MD040 (refs #250)

Three bare ``` fences in runbooks/opensearch-untagged-ri.md lacked a language
specifier, causing MD040 failures in CI. Added `text` to each (log-output and
ARN example blocks).

Author: cristim

runbooks/opensearch-untagged-ri.md

@@ -4,7 +4,7 @@
 
 Log pattern (grep / CloudWatch Logs Insights):
 
-```
+```text
 OPENSEARCH_TAG_FAILED commitment_id=<id> error=<msg>
 ```
 
@@ -18,7 +18,7 @@ AWS's `PurchaseReservedInstanceOffering` API has no inline `Tags` field.
 CUDly attempts a best-effort `AddTags` call after purchase using a
 constructed ARN of the form:
 
-```
+```text
 arn:aws:es:<region>:<account>:reserved-instance/<uuid>
 ```
 
@@ -38,7 +38,7 @@ this reservation unless it is manually tagged.
 
 1. Identify the untagged RI from the log line:
 
-   ```
+   ```text
    OPENSEARCH_TAG_FAILED commitment_id=<ri-uuid> error=...
    ```