haskellPackages.servant: fix sha256

My build server which isn't using cache.nixos.org discovered an
outdated hash in servant:

```
trying https://github.com/haskell-servant/servant/archive/v0.16.2.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   132    0   132    0     0    616      0 --:--:-- --:--:-- --:--:--   616
100  295k    0  295k    0     0   269k      0 --:--:--  0:00:01 --:--:--  384k
unpacking source archive /build/v0.16.2.tar.gz
hash mismatch in fixed-output derivation '/nix/store/i6qgxlqf599wl11rd44jasgmwb78wr6c-source':
  wanted: sha256:0kqglih3rv12nmkzxvalhfaaafk4b2irvv9x5xmc48i1ns71y23l
  got:    sha256:0xk3czk3jhqjxhy0g8r2248m8yxgvmqhgn955k92z0h7p02lfs89
```