README: fix mark-down mark-up

You know, I wanted to write this commit subject for a long time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
README: replace double-space after full stop with a single one

We are, after all, no longer in the age of the printing press.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>