Fix sidebar-mobile test to focus on an actual focusable element

Author: notriddle

src/test/rustdoc-gui/sidebar-mobile.goml

@@ -13,7 +13,7 @@ click: ".sidebar-menu"
 assert-css: (".sidebar-elems", {"display": "block", "left": "-246px"})
 // Force the sidebar open by focusing a link inside it.
 // This makes it easier for keyboard users to get to it.
-focus: ".sidebar-title"
+focus: ".sidebar-title a"
 assert-css: (".sidebar-elems", {"display": "block", "left": "0px"})
 // When we tab out of the sidebar, close it.
 focus: ".search-input"