Remove linux-nixpkgs configuration from README

This configuration no longer exists. We could define an empty
configuration with that name, but Bazel doesn't seem to support that
without a hack.

Author: mboes

README.md

@@ -254,26 +254,26 @@ This chooses the `cc_toolchain` bundled with GHC.
 ### Configuring your platform
 
 `rules_haskell` can be built and tested on Linux, MacOS, and Windows. Depending
-on the platform GHC can be provisioned using nixpkgs or by downloading a binary
+on the platform GHC can be provisioned using Nixpkgs or by downloading a binary
 distribution. In case of nixpkgs other toolchains (C compiler, Python, shell
 tools) will also be provided by nixpkgs, in case of bindist they will be taken
 from the environment (`$PATH`). The following `--config` options select the
 corresponding combination of operating system and GHC distribution:
 
 |                     |      Linux      |      MacOS      |      Windows      |
 | ------------------- | --------------- | --------------- | ----------------- |
-| nixpkgs             | `linux-nixpkgs` | `macos-nixpkgs` |                   |
-| binary distribution | `linux-bindist` | `macos-bindist` | `windows-bindist` |
+| Nixpkgs             | (unneeded)      | `macos-nixpkgs` | (unsupported)     |
+| Binary distribution | `linux-bindist` | `macos-bindist` | `windows-bindist` |
 
 Hint: You can use Bazel's `--announce_rc` flag to see what options are being
 used for a command in a specific configuration. E.g.
 ```
-$ bazel build //tests:run-tests --config linux-nixpkgs --nobuild --announce_rc
+$ bazel build //tests:run-tests --config linux-bindist --nobuild --announce_rc
 ```
 
 Hint: To avoid repetition you can add your configuration to `.bazelrc.local`.
 ```
-echo "build --config=linux-nixpkgs" >>.bazelrc.local
+echo "build --config=linux-bindist" >>.bazelrc.local
 ```
 
 ### Saving common command-line flags to a file