doc: correct technicality in chroot's --help output

* src/chroot.c (usage): Use correct quoting in descriptive diagnostic.
We would run `"$SHELL" -i`, not `${SHELL} -i`.

src/chroot.c

@@ -207,7 +207,7 @@ Run COMMAND with root directory set to NEWROOT.\n\
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
       fputs (_("\
 \n\
-If no command is given, run '${SHELL} -i' (default: '/bin/sh -i').\n\
+If no command is given, run '"$SHELL" -i' (default: '/bin/sh -i').\n\
 "), stdout);
       emit_ancillary_info (PROGRAM_NAME);
     }