pages/watch: HTML escape 'action' in download widget

Caught in the review of PR 5224, but forgot to click on "send review" in time.
I realized that too late, after the PR was already merged.

src/invidious/frontend/watch_page.cr

@@ -36,7 +36,7 @@ module Invidious::Frontend::WatchPage
     return String.build(4000) do |str|
       str << "<form"
       str << " class=\"pure-form pure-form-stacked\""
-      str << " action='#{url}'"
+      str << " action='" << HTML.escape(url) << "'"
       str << " method='post'"
       str << " rel='noopener noreferrer'"
       str << " target='_blank'>"