Fix: This implemention fails with the way of string construction.

[okcd00]

An example which fails in this implemention: "abb"

• The answer should be "bb".
• This implemention returns "b" only.
• With the fix, the answer is "bb" now.

Moreover, than is a typo, should be then.

Describe your change:

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

Checklist:

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

[okcd00]

All I modified is the line from manacher.py, but the tests failed in quantum/ripple_adder_classic.py.

It might be the remaining issue of other solutions, not for this PR.

==================================== ERRORS ====================================
_______________ ERROR collecting quantum/ripple_adder_classic.py _______________
quantum/ripple_adder_classic.py:6: in <module>
    from qiskit.providers import BaseBackend
E   ImportError: cannot import name 'BaseBackend' from 'qiskit.providers' (/opt/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/site-packages/qiskit/providers/__init__.py)

[cclauss]

In preparation for that denial-of-service attack that is Hacktoberfest, I am closing all pull requests that have failing tests.