backtracking -> word_search - replacing the example in doctest

[quant12345]

Describe your change:

Contributes to #9718

I replaced in this pull request 10188, the example in doctest with a less resource-intensive.

Before:

>>> word_exists([["A","A","A","A","A","A"],
...              ["A","A","A","A","A","A"],
...              ["A","A","A","A","A","A"],
...              ["A","A","A","A","A","A"],
...              ["A","A","A","A","A","B"],
...              ["A","A","A","A","B","A"]],
...             "AAAAAAAAAAAAABB")
False

After:

>>> word_exists([["B", "A", "A"], ["A", "A", "A"], ["A", "B", "A"]], "ABB")
False

• 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 description above includes the issue number(s) with a closing keyword: " #ISSUE-9718".

[tianyizheng02]

• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

@quant12345 Please include the issue number in your PR's description. This allows GitHub to automatically link your PR to the original issue. However, don't use the word "fixes", because your PR doesn't completely fix the issue on its own.