Fix typo in docstring

[aurorarossi]

This PR corrects a typo in the latex part of the docstring of the EGNNConv layer. It led to the following yellow line error:

[codecov[bot]]

Codecov Report

Merging #226 (d48dcb7) into master (a8863e3) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #226   +/-   ##
=======================================
  Coverage   86.58%   86.58%           
=======================================
  Files          15       15           
  Lines        1431     1431           
=======================================
  Hits         1239     1239           
  Misses        192      192           

Impacted Files	Coverage Δ
src/layers/conv.jl	76.78% <ø> (ø)

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[CarloLucibello]

thanks!