fix(backtracking): Correct doctest in m_coloring_problem

[sgindeed]

Describe your change:

This PR fixes a failing doctest in the solve function of the m-coloring problem algorithm. The test case for a simple 3-node graph with 2 available colors incorrectly expected a False result. Since this graph is bipartite and can be colored with two colors, the algorithm correctly returns True. This change updates the doctest to expect the correct True result, allowing the automated tests to pass.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• 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: "Fixes #ISSUE-NUMBER".