Add inAppPurchase to docs index

docs/README.md

@@ -65,6 +65,7 @@ an issue:
 * [contentTracing](api/content-tracing.md)
 * [dialog](api/dialog.md)
 * [globalShortcut](api/global-shortcut.md)
+* [inAppPurchase](api/in-app-purchase.md)
 * [ipcMain](api/ipc-main.md)
 * [Menu](api/menu.md)
 * [MenuItem](api/menu-item.md)

docs/api/in-app-purchase.md

@@ -1,4 +1,4 @@
-# inAppPurchase  _macOS_
+# inAppPurchase
 
 > In-app purchases on Mac App Store.