minor typo in API md

[pterrien]

Hi,

Minor but easy to fix: I noticed a typo in the API doc md: there are missing triple backquotes at the end of the Notifications section. This makes the following section, "Similar Artists", not displayed as expected.

Additionally, you may also want to add the "Similar Artists" section to the ToC.

Of course, I can provide a PR for this, even though it's mostly cosmetic I must admit.