doc: More uses of @lisp instead of @example.

* doc/guix.texi (G-Expressions): Use @lisp for 'let-system' example.
* doc/contributing.texi (Synopses and Descriptions): Use @lisp for
second example.

doc/contributing.texi

@@ -605,11 +605,11 @@ to make recommendations or instructions visible to them by inserting
 special comments like this (@pxref{xgettext Invocation,,, gettext, GNU
 Gettext}):
 
-@example
+@lisp
 ;; TRANSLATORS: "X11 resize-and-rotate" should not be translated.
 (description "ARandR is designed to provide a simple visual front end
 for the X11 resize-and-rotate (RandR) extension. @dots{}")
-@end example
+@end lisp
 
 @node Snippets versus Phases
 @subsection Snippets versus Phases

doc/guix.texi

@@ -9396,7 +9396,7 @@ cross-compiling.
 @code{let-system} is useful in the occasional case where the object
 spliced into the gexp depends on the target system, as in this example:
 
-@example
+@lisp
 #~(system*
    #+(let-system system
        (cond ((string-prefix? "armhf-" system)
@@ -9406,7 +9406,7 @@ spliced into the gexp depends on the target system, as in this example:
              (else
               (error "dunno!"))))
    "-net" "user" #$image)
-@end example
+@end lisp
 @end deffn
 
 @deffn {Scheme Syntax} with-parameters ((@var{parameter} @var{value}) @dots{}) @var{exp}