Documentation: replace ~/.bup with $BUP_DIR in bup-index.md and bup-save.md.

When one is not using the default bupdir placement, then those files
aren't found inside ~/.bup, but wherever "-d /path.." or $BUP_DIR are
pointing to.

Thanks to krichter722@aol.de for mentioning this.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>