Correct fish shell usage documentation

[dallbee]

Fish shell uses &| for combined stderr and stdout piping, not ^|.

I think ^| was used in older (2.xx) versions of fish, but we fish users aren't exactly the type to keep an old version around 😄

[codecov]

Codecov Report

Merging #58 (9687baa) into master (1c1d1eb) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master     #58     +/-   ##
========================================
- Coverage    90.1%   90.0%   -0.1%     
========================================
  Files          12      12             
  Lines        3464    3464             
========================================
- Hits         3120    3118      -2     
- Misses        280     281      +1     
- Partials       64      65      +1     

Impacted Files	Coverage Δ
stack/stack.go	84.4% <0.0%> (-0.8%)	⬇️

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[maruel]

Thanks!

Looks like I have to update the CI. :/