Fixed small typos in Thingsboard widget code

thingsboard/custom_image_view_widget.html

 <div>
     <h1>Live Image from Device:</h1>
-    <p>Last updated: ${rpiCameraImageTimestamp}</p>
-    <img src="data:image/jpeg; base64, ${rpiCameraImage}" />
+    <p>Last updated: {rpiCameraImageTimestamp}</p>
+    <img src="data:image/jpeg; base64, {rpiCameraImage}" />
 </div>
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