fixed typo

.appveyor.yml

@@ -1,12 +1,13 @@
+image: Visual Studio 2017
 environment:
   matrix:
     - nodejs_version: "6"
     - nodejs_version: "7"
     - nodejs_version: "8" 
     - nodejs_version: "9"
-  platform:
-    - x86
-    - x64
+platform:
+  - x86
+  - x64
 install:
   - ps: Install-Product node $env:nodejs_version $env:platform
   - npm install