Add clearer link to nightly builds in Github repo.
Add clearer link to nightly builds in Github repo.

Addresses GitHub issue #417
https://github.com/tlaplus/tlaplus/issues/417

[Documentation]

lemmy/tlaplus,lemmy/tlaplus,lemmy/tlaplus,lemmy/tlaplus,tlaplus/tlaplus,tlaplus/tlaplus,tlaplus/tlaplus,tlaplus/tlaplus