Use full path of span

Co-authored-by: Chris Simpkins <git.simpkins@gmail.com>

Author: mark-i-m

src/macro-expansion.md

@@ -222,7 +222,7 @@ The context is attached to AST nodes. All AST nodes generated by macros have
 context attached. Additionally, there may be other nodes that have context
 attached, such as some desugared syntax (non-macro-expanded nodes are
 considered to just have the "root" context, as described below).
-Throughout the compiler, we use [`Span`s][span] to refer to code locations.
+Throughout the compiler, we use [`librustc_span::Span`s][span] to refer to code locations.
 This struct also has hygiene information attached to it, as we will see later.
 
 [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html