message: Fix readme Docker commands for Windows
  A very minor quality of life improvement: the readme's Docker commands can almost be copy/pasted into a PowerShell prompt, but `$PWD:` needs to be `${PWD}:`.
diff: @@ -177,7 +177,7 @@ Tarpaulin has builds deployed to [docker-hub](https://hub.docker.com/r/xd009642/
  to run Tarpaulin on any system that has Docker, run this in your project directory:
  ```text
  -docker run --security-opt seccomp=unconfined -v "$PWD:/volume" xd009642/tarpaulin
  +docker run --security-opt seccomp=unconfined -v "${PWD}:/volume" xd009642/tarpaulin
  ```
  This builds your project inside Docker and runs Tarpaulin without any arguments. There are
  @@ -186,14 +186,14 @@ versions after 0.5.6 will have the latest release built with the rust stable and
  compilers. To get the latest development version built with rustc-nightly run the following:
  ```text
  -docker run --security-opt seccomp=unconfined -v "$PWD:/volume" xd009642/tarpaulin:develop-nightly
  +docker run --security-opt seccomp=unconfined -v "${PWD}:/volume" xd009642/tarpaulin:develop-nightly
  ```
  Note that the build might fail if the Docker image doesn't contain any necessary
  dependencies. In that case, you can install dependencies before, like this:
  ```text
  -docker run --security-opt seccomp=unconfined -v "$PWD:/volume" xd009642/tarpaulin sh -c "apt-get install xxx && cargo tarpaulin"
  +docker run --security-opt seccomp=unconfined -v "${PWD}:/volume" xd009642/tarpaulin sh -c "apt-get install xxx && cargo tarpaulin"
  ```
  ## Extending Tarpaulin.
  