Documentation fixes for GATv2Conv

[Animiral]

As well as the change to addition, added a code comment about equivalence to the GATv2 paper.

Resolve #175.

[codecov]

Codecov Report

Merging #176 (62111f0) into master (aa755dd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   86.01%   86.01%           
=======================================
  Files          15       15           
  Lines        1323     1323           
=======================================
  Hits         1138     1138           
  Misses        185      185           

Impacted Files	Coverage Δ
src/layers/conv.jl	77.31% <100.00%> (ø)

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[CarloLucibello]

thanks!