docs: fix link to TRExFitter documentation

[alexander-held]

Description

The TRExFitter part of the translation section in the documentation contains a link to ATLAS-internal documentation. Following a restructuring of that internal documentation, the link used on the pyhf website is no longer correct. This fixes the link.

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• Tests are passing
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* Update URL to 'TRExFitter to pyhf' translation docs
   - c.f. https://trexfitter-docs.web.cern.ch/trexfitter-docs/interfacing_tools/pyhf/

[codecov]

Codecov Report

Merging #1777 (de51956) into master (c2c2891) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1777   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4270     4270           
  Branches      683      683           
=======================================
  Hits         4190     4190           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.58% <ø> (ø)
unittests	96.18% <ø> (ø)

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[matthewfeickert]

LGTM. 👍 Thanks!