rustdoc: Bind keydown instead of keypress for nav

Apparently keypress doesn't quite work in all browsers due to some not invoking
the handler and jquery not setting the right `which` field in all circumstances.
According to http://stackoverflow.com/questions/2166771 switching over to
`keydown` works and it appears to do the trick. Tested in Safari, Firefox, and
Chrome.

Closes #15011

src/librustdoc/html/static/main.js

@@ -357,8 +357,8 @@
                 }, 20);
             });
 
-            $(document).off('keypress.searchnav');
-            $(document).on('keypress.searchnav', function(e) {
+            $(document).off('keydown.searchnav');
+            $(document).on('keydown.searchnav', function(e) {
                 var $active = $results.filter('.highlighted');
 
                 if (e.which === 38) { // up