receipt: Fix display of notes field

Unlike the normal `<textarea>` element, `<sl-textarea>` uses a `value`
attribute to store the prefilled value of the field, rather than a child
text node.

templates/receipt.html.tera

@@ -60,7 +60,12 @@
       ></sl-input>
     </p>
     <p>
-      <sl-textarea label="Notes" name="notes" readonly>{{ notes }}</sl-textarea>
+      <sl-textarea
+        label="Notes"
+        name="notes"
+        value="{{ notes }}"
+        readonly
+      ></sl-textarea>
     </p>
   </div>
   <div class="photo">