gatsby-remark-autolink-headers document the offsetY option

[pradel]

Description

Actually in the gatsby-remark-autolink-headers docs the offsetY option is not documented I had to search in the source code to find it.
I am opening this issue as a reminder, Will try to submit a pr when I have some time :)

[m-allanson]

Done in #5247, thanks @RobinCsl.