tiny change to docstring of gpu function

[GSmithApps]

Changed the docstring signature of the gpu function from gpu(x) to gpu(m) to conform with cpu and to fit the accompanying sentence in the docstring.

This is my first open source PR ever... forgive me for it being so tiny... gotta start somewhere. Cheers to Flux being cool

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -2.81 ⚠️

Comparison is base (484796c) 82.63% compared to head (16d0ff5) 79.83%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2206      +/-   ##
==========================================
- Coverage   82.63%   79.83%   -2.81%     
==========================================
  Files          23       23              
  Lines        1578     1577       -1     
==========================================
- Hits         1304     1259      -45     
- Misses        274      318      +44     

Impacted Files	Coverage Δ
src/functor.jl	37.34% <ø> (-37.35%)	⬇️
src/cuda/cudnn.jl	0.00% <0.00%> (-100.00%)	⬇️
src/layers/normalise.jl	93.79% <0.00%> (-1.38%)	⬇️
src/layers/conv.jl	88.95% <0.00%> (-0.07%)	⬇️
src/Flux.jl	0.00% <0.00%> (ø)

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[ToucheSir]

Good catch, thanks for your (first) PR!