Minor CSS: adjust access captcha height

It was too high on mobile screens.

anonstream/templates/captcha.html

@@ -15,7 +15,7 @@
       font-family: sans-serif;
       font-size: 14pt;
       display: grid;
-      grid-template-rows: calc(50% - 4rem) 1fr;
+      grid-template-rows: calc(50% - 10vh + 2rem) 1fr;
       height: 100vh;
       margin: 0;
       padding: 1rem;