rename "submit" button to "Generate preview"

templates/image.html

@@ -5,7 +5,7 @@
     <h1>Upload Image</h1>
     <form method="POST" action="/image" enctype="multipart/form-data">
         <input type="file" name="image">
-        <input type="submit" value="Upload">
+        <input type="submit" value="Generate preview">
     </form>
 
     {% if filename %}

templates/text.html

@@ -29,7 +29,7 @@
         style="width: 40px"><br>
         Y position:  <input type="number" value="{{text2["pos"]}}" name="pos2" required
         style="width: 40px"><br>
-        <input type="submit" value="Submit">
+        <input type="submit" value="Generate preview">
     </form>
 
     {% if filename %}