doc: Shrink width of deeply-indented code sample.

This follows up on commit 4cafdce210.

* doc/guix.texi (operating-system Reference): Shorten comments to fit
everything on an 80-character punch card.

doc/guix.texi

@@ -11967,9 +11967,9 @@ at @file{/run/current-system/profile}.  Each element is either a package
 variable or a package/output tuple.  Here's a simple example of both:
 
 @lisp
-(cons* git                               ; only the default "out" output
-       (git "send-email")                ; another output of the git package
-       %base-packages)                   ; the default set
+(cons* git                ; the default "out" output
+       (git "send-email") ; another output of git
+       %base-packages)    ; the default set
 @end lisp
 
 The default set includes core utilities and it is good practice to