Fixed shortcut handling. Reverted to [Shift]+[+=]

Realized browsers use [Ctrl]+[+=] for zoom, so using [Shift]+[+=] for
collapse/expand was not necessarily a conflict.

Also a bugfix.

src/librustdoc/html/static/main.js

@@ -124,8 +124,8 @@
             focusSearchBar();
             break;
 
-        case "t":
-        case "T":
+        case "+":
+            ev.preventDefault();
             toggleAllDocs();
             break;