Fix LaTeX rendering in r1cs_proof.

[ndcroos]

There is an issue with the LaTeX rendering here: https://doc-internal.dalek.rs/bulletproofs/notes/r1cs_proof/index.html#variables

Perhaps other documentation pages are also affected.

[rubdos]

Related: dalek-cryptography/curve25519-dalek#372

zkcrypto/curve25519-dalek-ng@7ba81d7