rustdoc: ayu code color selector more specific

According to https://github.com/rust-lang/rust/pull/100960#issuecomment-1225970579,
this selector is only really intended to apply to item info. However,
it's so broad that it's hard to tell when it deliberately applies vs where it
accidentally applies.

src/librustdoc/html/static/css/themes/ayu.css

@@ -47,7 +47,7 @@ h4 {
 .docblock pre > code, pre > code {
 	color: #e6e1cf;
 }
-span code {
+.item-info code {
 	color: #e6e1cf;
 }
 .docblock a > code {