Fix a couple xref links

They were still using the old ezdoc syntax.

doc/src/guide/req.asciidoc

@@ -274,7 +274,7 @@ parameter `key` was defined, but with no value.
 
 The second function Cowboy provides allows you to match out
 only the parameters you are interested in, and at the same
-time do any post processing you require using ^constraints^.
+time do any post processing you require using xref:constraints[constraints].
 This function returns a map.
 
 [source,erlang]

doc/src/guide/req_body.asciidoc

@@ -15,7 +15,7 @@ present and was not read, Cowboy will either cancel or
 skip its download, depending on the protocol.
 
 Cowboy provides functions for reading the body raw,
-and read and parse form urlencoded or ^multipart^ bodies.
+and read and parse form urlencoded or xref:multipart[multipart bodies].
 The latter is covered in its own chapter.
 
 === Request body presence