Fix responsiveness of header (graphql#957)

Author: carolstran

src/assets/css/_css/graphql.less

@@ -119,7 +119,7 @@ p code {
 
 header {
   .headline-font(@size: 15px);
-  height: 50px;
+  min-height: 50px;
   background: white;
   box-shadow: inset 0 -1px 0 0px rgba(0, 0, 0, 0.1);
   z-index: 10;