fix(README): add language to fenced code block (MD040)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: alpeb

README.md

@@ -237,7 +237,7 @@ Push all images:
 A minimal number of people have access to push new Linkerd dev containers.
 To modify the user with this permission, navigate to:
 
-```
+```text
 linkerd > Packages > dev
 ```