fix documentation for clang11Stdenv dev shell

[peterbecich]

Motivation

Changes references in documentation from clang11StdenvPackages to clang11Stdenv

This works:

$ nix develop .#clang11Stdenv   

peterbecich@neptune:~/nix/nix$ 

Context

clang11StdenvPackages does not exist

$ nix flake show
.
.
.
devShells
.
.
.
│   └───x86_64-linux
│       ├───ccacheStdenv: development environment 'nix'
│       ├───clang11Stdenv: development environment 'nix'
│       ├───clangStdenv: development environment 'nix'
│       ├───default: development environment 'nix'
│       ├───gccStdenv: development environment 'nix'
│       ├───libcxxStdenv: development environment 'nix'
│       └───stdenv: development environment 'nix'

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes