fix dappnodepp images (#491)

Author: Marketen

docs/user/dappnode-app/how-to-install.md

@@ -39,17 +39,17 @@ The following installation process is based on the **Chrome mobile and desktop e
 
 2. Click the **Install App** button and then click Install in the pop up. 
 <p align="center">
-    <img src="../../../img/dnp-app-mobile.png"/>
+    <img src="/img/dnp-app-mobile.png" alt="dnpappmobile"/>
 </p>
 
 3. The App will open in a new window and you need to click **Grant permissions** button. In the following browser pop up click **Allow**.
 <p align="center">
-    <img src="../../../img/grant-mobile.png"/>
+    <img src="/img/grant-mobile.png" alt="grantmobile"/>
 </p>
 
 4. The app was **succesfully configured**. You can manage your subscribed devices and test the notifications.
 <p align="center">
-    <img src="../../../img/devices-mobile.png"/>
+    <img src="/img/devices-mobile.png" alt="devicesmobile"/>
 </p>