Fixed typo in matrix attention layer (should be Z instead of X).

[benedict-96]

Changes are $X \to Z$ where the wrong letter was used previously.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.82%. Comparing base (a9c1589) to head (aeacd0e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #213   +/-   ##
=======================================
  Coverage   61.82%   61.82%           
=======================================
  Files         124      124           
  Lines        3790     3790           
=======================================
  Hits         2343     2343           
  Misses       1447     1447           

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