Readme build instructions use the incorrect directory

[ScottWales]

Describe the bug

The readme file's build instructions say

(Recommended) Remove the existing {{ cookiecutter.book_name }}/_build/ directory
Run jupyter-book build {{ cookiecutter.book_name }}/

however the directory is actually named {{cookiecutter.book_slug}}.

To Reproduce

Steps to reproduce the behavior:

Create a book where the book name and slug are different and follow the build instructions in the generated readme

Expected behavior

A clear and concise description of what you expected to happen.

Readme should read

(Recommended) Remove the existing {{ cookiecutter.book_slug }}/_build/ directory
Run jupyter-book build {{ cookiecutter.book_slug }}/

Environment

• Python Version [e.g. 3.7.1]: 3.8.6
• Package versions or output of jupyter-book --version:

  Jupyter Book: 0.10.1
  MyST-NB: 0.12.0
  Sphinx Book Theme: 0.0.41
  MyST-Parser: 0.13.5
  Jupyter-Cache: 0.4.2
  NbClient: 0.5.2

• Operating System: WSL

Additional context

Add any other context about the problem here.

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[TomasBeuzen]

Thanks for this @ScottWales - I'll make a PR later tonight to fix this one up. Appreciate it!