README: normalize indentation of verbatim blocks

[waldyrious]

While I am aware that there has been an explicit choice not to make the README strictly formatted as markdown (#22), the current content is already effectively markdown-compatible (the headings and the code/verbatim/preformatted blocks, in particular).

Since the existing preformatted blocks already use 4-spaces indentation (which is what makes them markdown-ready), it seems appropriate to also apply the same indentation to the last such block in the file.

[waldyrious]

Note: There a couple additional minor changes that would benefit a markdown-style rendering of the text even, without adding the .md extension.

One is adding brackets to link labels:

- foobar (https://example.com)
+[foobar](https://example.com)

...but I feared that doing so might be going too far if the intent is to make the file readable primarily in plaintext mode.

The second one is using backticks around variables, commands and file paths. I do believe this might be a subtle enough change (which would also allow sentences to be read more naturally even in plaintext mode) to be seriously considered. If that would be acceptable, I'd be happy to add the changes to this PR.