Fix invalid LaTeX macros for mu, nu, tau, and down-arrow in equation conversion

[ksprajapatii]

What

Fixes typo'd LaTeX macros in converter_utils/docx/math/latex_dict.py
that produced invalid LaTeX output when converting Word equations
containing certain symbols.

Bugs fixed

• \m → \mu (U+1D707, mathematical italic mu)
• \n → \nu (U+1D708, mathematical italic nu)
• \ta → \tau (U+1D70F, mathematical italic tau)
• \downright → \downarrow (U+2193, downwards arrow)

\m, \n, \ta, and \downright are not valid LaTeX commands. Any
.docx file with an equation containing μ, ν, τ, or ↓ would produce
broken/invalid LaTeX in the converted markdown output.

How I found it

Ran ruff static analysis over the codebase, which flagged a
duplicate dict key warning near these entries. Manual review of the
surrounding Unicode-to-LaTeX mappings revealed the typos.

Testing

• Verified the corrected dict values directly:
  T['\U0001d707'] == '\\mu ', etc.
• Existing test_docx_equations test in test_module_misc.py still
  passes.
• Added a small regression test to catch these mappings from
  regressing in future.

[ksprajapatii]

@microsoft-github-policy-service agree

[afourney]

Merged! Thanks!