Fix size of donate button.

timhoffm · timhoffm · commit e8be16e0f2fe · 2020-07-06T02:34:42.000+02:00
This was a regression of #17825.
diff --git a/doc/_static/mpl.css b/doc/_static/mpl.css
@@ -1029,7 +1029,6 @@ figcaption {
 .donate_button {
     clear: both;
     display: block;
-    width: 200px;
     margin: 30px auto 0;
 }
 
