Fix search hotkey

https://github.com/rust-lang/mdBook/pull/2608 accidentally broke the
search "S" keybinding by using the wrong capitalization.

src/front-end/searcher/searcher.js

@@ -356,7 +356,7 @@ window.search = window.search || {};
             }
             showSearch(false);
             marker.unmark();
-        } else if (!hasFocus() && (e.key === 'S' || e.key === '/')) {
+        } else if (!hasFocus() && (e.key === 's' || e.key === '/')) {
             e.preventDefault();
             showSearch(true);
             window.scrollTo(0, 0);

tests/gui/search.goml

@@ -7,7 +7,7 @@ define-function: (
     [],
     block {
         assert-css: ("#search-wrapper", {"display": "none"})
-        press-key: 'S'
+        press-key: 's'
         wait-for-css-false: ("#search-wrapper", {"display": "none"})
     }
 )