fixed link format

[RyanKung]

Description

Use an automatic link instead.

When I build the doc with cargo doc --open (cargo 1.51.0-nightly (783bc43c6 2021-01-20)), I got err msg like:

error: this URL is not a hyperlink
  --> src/groups/curves/short_weierstrass/mod.rs:13:21
   |
13 | /// the [\[BLS12]\](https://eprint.iacr.org/2002/088.pdf) family of bilinear groups.
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://eprint.iacr.org/2002/088.pdf>`
   |
note: the lint level is defined here
  --> src/lib.rs:5:5
   |
5  |     warnings,
   |     ^^^^^^^^
   = note: `#[deny(non_autolinks)]` implied by `#[deny(warnings)]`

error: this URL is not a hyperlink
  --> src/groups/curves/short_weierstrass/mod.rs:17:20
   |
17 | /// the [\[MNT4]\](https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.20.8113&rep=rep1&type=pdf)
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.20.8113&rep=rep1&type=pdf>`

error: this URL is not a hyperlink
  --> src/groups/curves/short_weierstrass/mod.rs:21:20
   |
21 | /// the [\[MNT6]\](https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.20.8113&rep=rep1&type=pdf)
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.20.8113&rep=rep1&type=pdf>`

error: aborting due to 3 previous errors

error: could not document `ark-r1cs-std`

[Pratyush]

Thanks for the fix!