Syntax error in mermaid scheme on concept page

[ivanovmg]

Bug report

What's wrong

When I go to https://classes.readthedocs.io/en/latest/pages/concept.html I can see that the scheme, which follows the sentence "To use typeclasses you should understand these steps:" is not shown properly.

How is that should be

It should show the scheme without "Syntax error" highlight.

Syntax error in https://github.com/dry-python/classes/blob/master/docs/pages/concept.rst#steps should probably be fixed.

[sobolevn]

PR is welcome!