chroot: fix typo in preceding change: didn't compile

* src/chroot.c (usage): Add backslashes.

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);
     }