[Bug-mit-scheme] vector-append missing for docs

[Abhinav Jauhri]

Hey,

I was going through the documentation for vectors
(https://www.gnu.org/software/mit-scheme/documentation/mit-scheme-ref/Vectors.html#Vectors)
and realized there is no mention of `vector-append` method. IMO it
should probably exist here
(https://www.gnu.org/software/mit-scheme/documentation/mit-scheme-ref/Construction-of-Vectors.html#Construction-of-Vectors)

Thanks!

- Abhinav Jauhri