Fix YAML error in deprecation_request.yaml (#25641)

Apparently "@" is a reserved token when it is the start of a YAML value.

Would have been nice if GitHub had any way to preview or validate before
merging. 😅

Author: xirzec

.github/ISSUE_TEMPLATE/deprecation_request.yaml

@@ -14,7 +14,7 @@ body:
     attributes:
       label: Package Name
       description: What package would you like to deprecate?
-      placeholder: @azure/example
+      placeholder: "@azure/example"
     validations:
       required: true
   - type: textarea