Fix removing the root node in binary_search_tree.py removes the whole tree

[Frank-1998]

Describe your change:

Fixes: #8715
Fixes: #8819

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[tianyizheng02]

Fixes #8715

Apparently it doesn't work unless the creator of the PR does it... @Frank-1998 could you add that to your PR description so that GitHub automatically links the issue?

[Frank-1998]

@tianyizheng02 Sorry, could you please explain what should I add here? I am a bit confused. Thank you.

[tianyizheng02]

@Frank-1998 You should add Fixes #8715 to your PR description so that GitHub links this PR to the issue that it fixes. Here's an example of another PR to show you what it should look like:

Once you do that, you should see something like this on the right side of the page:

Once you do this, GitHub will automatically close the linked issue when this PR is approved and merged.

[Frank-1998]

@tianyizheng02 Oh, I see, it's done, thank you very much. :)