Append missing '",' in the config file

Bug: n/a
Test: n/a
Change-Id: I677434476b2d6ee3cb997f1916ed53b0d7839eac
diff --git a/docs/custom_vm.md b/docs/custom_vm.md
index e4b374f..3d434ed 100644
--- a/docs/custom_vm.md
+++ b/docs/custom_vm.md
@@ -53,7 +53,7 @@
     "name": "debian",
     "disks": [
         {
-            "image": "/data/local/tmp/debian.img
+            "image": "/data/local/tmp/debian.img",
             "partitions": [],
             "writable": true
         }