Change default donation amount to $10.

ECOM-2113

lms/templates/dashboard/donation.underscore

@@ -1,7 +1,7 @@
 <div class="donation-error-msg" />
 <form class="nav-item donate-form">
     <span class="monetary-symbol">$</span>
-    <input class="amount" type="text" name="amount" value="5" />
+    <input class="amount" type="text" name="amount" value="10" />
     <button class="btn action-primary action-donate" type="submit" name="Donate"><%- gettext('Donate') %></button>
 </form>
 <form class="payment-form"></form>