gh-35382: Add missing backticks to correct formatting
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### :books: Description
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`matrix_modn_dense_float.pyx` had single backticks in one of the
docstrings which was causing it to render as LaTeX text, while it was
intended to be monospace formatted. The missing backticks are added to
fix this.
Fixes #35363
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URL: https://github.com/sagemath/sage/pull/35382
Reported by: Karan Handa
Reviewer(s): Matthias Köppe
