docs: fix JSON format error

Signed-off-by: arcosx <arcosx@outlook.com>

image/spec/v1.2.md

@@ -142,7 +142,7 @@ Here is an example image JSON file:
         ],
         "Volumes": {
             "/var/job-result-data": {},
-            "/var/log/my-app-logs": {},
+            "/var/log/my-app-logs": {}
         },
         "WorkingDir": "/home/alice"
     },