Wrong headings in the list of intrinsics of mve.md

[fpetrogalli]

@ValeriaDaneva, I have missed something in my review of #29.

I have manually created a PDF version of the MVE intrinsic specifications produced out of the changes in #29 and noticed that there is an issue with the headings of the list of intrinsics. To reproduce, run the following command:

pandoc --toc -o mve.pdf -V geometry:landscape  -V geometry:margin=1cm mve.md

You will notice that the sections in the List of intrinsics section need to be moved down in the hierarchy of the headings. this issue is visible also in the HTML output of github.io: https://arm-software.github.io/acle/mve_intrinsics/mve.html#list-of-intrinsics

[fpetrogalli]

Fixed by #42