add quotes

Author: Qinghao Shi

.github/workflows/snippets_checks.yml

@@ -24,7 +24,7 @@ jobs:
 
       - name: add mbed_app.json
         run: |
-          echo  { "target_overrides": { "K64F": { "target.features_add": ["BLE"] } } } > mbed_app.json
+          echo  '{ "target_overrides": { "K64F": { "target.features_add": ["BLE"] } } }' > mbed_app.json
 
       - name: validate code snippets
         run: ./check_tools/find_bad_code_snippets.sh