doc: Fix typo in example.

* doc/guix.texi (Invoking guix deploy): Fix example to use '%user'
variable defined above.

Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>

doc/guix.texi

@@ -27028,7 +27028,7 @@ be accomplished with the following operating system configuration snippet:
      (plain-file "sudoers"
                  (string-append (plain-file-content %sudoers-specification)
                                 (format #f "~a ALL = NOPASSWD: ALL~%"
-                                        %username)))))
+                                        %user)))))
 @end lisp