docs: fix broken paths to docs/devel/qapi-code-gen.txt

With the move of some docs to docs/interop on ac06724a71,
a couple of references were not updated.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qapi/introspect.json

@@ -226,7 +226,7 @@
 #
 # @members: the alternate type's members, in no particular order.
 #           The members' wire encoding is distinct, see
-#           docs/qapi-code-gen.txt section Alternate types.
+#           docs/devel/qapi-code-gen.txt section Alternate types.
 #
 # On the wire, this can be any of the members.
 #