readme: Fix typo (“prequal” → “prequel”)

In the `readme.md` document, fix a typo introduced by vultraz.

Specifically, the correction is: “prequal” → “prequel”.
readme: Correct Markdown formatting

In the `readme.md` document, correct a Markdown formatting error —
specifically, that an unordered list and the preceding paragraph, both
introduced in vultraz’s rewrite of the document, were not separated by a
blank line, as is generally required.