Update docs path

[sbc64]

When I ran cargo doc --open I would get this error:

 Documenting ethers v0.1.0 (/home/sebas/repos/eth2.0/ethers-rs/ethers)
error: `[secp256k1]` cannot be resolved, ignoring it.
       --> ethers/src/lib.rs:250:58
       |
250 | /// under the `abi` module, as well as the [`secp256k1`](secp256k1) and [`rand`](rand)
       |                                                                                          ^^^^^^^^^ cannot be resolved, ignoring
       |
...
error: `[rand]` cannot be resolved, ignoring it.
      --> ethers/src/lib.rs:250:82
       |
250 | /// under the `abi` module, as well as the [`secp256k1`](secp256k1) and [`rand`](rand)
       |                                                                                                                                ^^^^ cannot be resolved, ignoring
       |
      = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

error: aborting due to 2 previous errors

error: Could not document `ethers`.

This PR changes the docs url paths for both secp256k1 and rand. It is using the docs.rs docs but notice that the version of rand is not matching with Cargo.toml. 0.7.3 in the docs.rs url vs 0.7.2 used in Cargo). I used the docs.rs url so that it is consistent with the ethabi link in the same comment location.

[gakonst]

LGTM. Thanks!