Update SPIFFE-ID.md
Update SPIFFE-ID.md

While I was trying to point to one specific section of this doc I noticed that the links in the TOC are broken.
Dots (.) won't work when trying to generate an anchor link.
For example:
Doesn't work
https://github.com/spiffe/spiffe/blob/master/standards/SPIFFE-ID.md#2.1.-trust-domain
Works
https://github.com/spiffe/spiffe/blob/master/standards/SPIFFE-ID.md#21-trust-domain
spiffe/spiffe