Show backticks in testing-doctest.md (#4941) (#4954)

(cherry picked from commit c527afc)

Signed-off-by: Lantao Jin <ltjin@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

docs/dev/testing-doctest.md

@@ -75,12 +75,15 @@ For PPL documentation, Markdown format is now supported with the following guide
 
 3. **Code Block Format**: Use **paired** fenced code blocks - each input block must be followed by its expected output block:
 
+````text
 ```ppl
 search source=accounts | where age > 25 | fields firstname, lastname
 ```
+````
 
 Expected output:
 
+````text
 ```text
 +-------------+------------+
 | firstname   | lastname   |
@@ -89,22 +92,19 @@ Expected output:
 | Hattie      | Bond       |
 +-------------+------------+
 ```
+````
 
 **Input/Output Pairs**: Each input code fence must be immediately followed by an "Expected output:" section with an output code fence
 - **Supported Input Languages**: `sql`, `ppl`, `bash`, `sh`, `bash ppl`
 - **Supported Output Languages**: `text`, `console`, `output`, `json`, `yaml`
 
 4. **Ignoring Tests**: To skip specific code blocks from testing, add `ignore` attribute:
 
+````text
 ```ppl ignore
 search source=accounts | head 5
 ```
-
-Expected output:
-
-```text
-This output won't be tested
-```
+````
 
 5. **Validation**: Markdown categories only accept `.md` files - mixing with `.rst` files will cause validation errors
 6. **Testing**: Run `./gradlew doctest` to validate your markdown documentation