rustdoc: CSS prevent sidebar width change jank

This commit makes the `width` and `min-width` of the sidebar the same. They
originally were when cad0fce205 added the
`min-width` rule, but 6a5f8b1aef changed the
width without changing the `min-width`, causing it to sometimes oscilate
between 200 and 250 pixels depending on the main content.

src/librustdoc/html/static/css/rustdoc.css

@@ -411,7 +411,7 @@ img {
 
 .sidebar {
 	font-size: 0.875rem;
-	width: 250px;
+	width: 200px;
 	min-width: 200px;
 	overflow-y: scroll;
 	position: sticky;