build: fix JSON formatting for --print

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/bake-reference.md

@@ -328,7 +328,9 @@ $ docker buildx bake "https://github.com/docker/cli.git#v20.10.11" --print
 {
   "group": {
     "default": {
-      "targets": ["binary"]
+      "targets": [
+        "binary"
+      ]
     }
   },
   "target": {
@@ -341,8 +343,12 @@ $ docker buildx bake "https://github.com/docker/cli.git#v20.10.11" --print
         "VERSION": ""
       },
       "target": "binary",
-      "platforms": ["local"],
+      "platforms": [
-      "output": ["build"]
+        "local"
+      ],
+      "output": [
+        "build"
+      ]
     }
   }
 }