Fix tab in change log

The tab character doesn't work in every version of `sed` (`\t` gets displayed as `t` on my mac).
See http://stackoverflow.com/questions/6363328/replacing-with-tab-using-sed

The proposed fix (typing `<Ctrl>+V <tab>` in the terminal window) doesn't work in case of copy-pasting the command to the terminal, so I will just replace this tab by a space.