README: fix block syntax according to the four-space rule

[rohieb]

It seems that GitHub interprets the indentation here different than other Markdown parsers, which always count multiples of four spaces as indentation. GitHub also accepts three spaces as indentation for the list item. To be compatible with other Markdown implementations, apply the four-space rule.

The code block here is inside a list block, so it must be indented by four spaces more than the list block.