ota-wizard: fix grid layout

src/pages/ota-wizard/ota-wizard.html

@@ -14,7 +14,7 @@
     <ion-slide>
       <ion-grid>
         <ion-row>
-          <ion-col col-lg-6>
+          <ion-col col-12 col-md-6>
             <ion-icon name="wifi" style="font-size: 160px"></ion-icon>
             <h2>Welcome</h2>
             <p>
@@ -23,7 +23,7 @@
             </p>
           </ion-col>
 
-          <ion-col col-lg-6>
+          <ion-col col-12 col-md-6>
             <p>First, please make sure that your senseBox...</p>
             <ul style="text-align: left">
               <li>has the <b>WiFi shield</b> plugged in</li>