Fix capitalisation of Makefile in CODEOWNERS

[thomasspriggs]

This PR fixes the capitalisation of Makefile in CODEOWNERS. Additional reviewers were being required, because the lower case rule was not being matched against the actual file name.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6466 (578765e) into develop (113bcdd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6466   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1542     1542           
  Lines       165038   165038           
========================================
  Hits        125404   125404           
  Misses       39634    39634           

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