doc: Correct short name of `--install' option of `guix-package'.

* doc/guix.texi (Invoking guix-package): Fix short form of `--install'.
  Reported by Andreas Enge <andreas@enge.fr>.

doc/guix.texi

@@ -210,7 +210,7 @@ want to roll back.
 @table @code
 
 @item --install=@var{package}
-@itemx -x @var{package}
+@itemx -i @var{package}
 Install @var{package}.
 
 @var{package} may specify either a simple package name, such as