doc: Link to "Building from Git" instead of quoting command to build Guix.

* doc/contributing.texi (Submitting Patches): Remove example.
Add link to "Building from Git" section.

doc/contributing.texi

@@ -962,11 +962,8 @@ the URL: it is not very useful and if the name changes, the URL will probably
 be wrong.
 
 @item
-See if Guix builds with
-@example
-guix environment --pure guix -- make
-@end example
-and look for warnings, especially those about use of undefined symbols.
+Check if Guix builds (@pxref{Building from Git}) and address the
+warnings, especially those about use of undefined symbols.
 
 @item
 Make sure your changes do not break Guix and simulate a @code{guix pull} with: