[mathcore] Fix return of FitResult::ParameterBounds as doc says.

[Axel-Naumann]

This Pull request:

Changes or fixes:

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #

[sonatype-lift]

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[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac11/noimt, mac12arm/cxx20, windows10/cxx14
How to customize builds

[github-actions]

Test Results

         9 files           9 suites   1d 21h 37m 6s ⏱️
  2 472 tests   2 472 ✔️ 0 💤 0 ❌
21 147 runs  21 147 ✔️ 0 💤 0 ❌

Results for commit 3262fc4.

[phsft-bot]

Build failed on ROOT-debian10-i386/soversion.
Running on pcepsft11.dyndns.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.root.io.json.roottest_root_io_json_make

[lmoneta]

LGTM!
Thank you Axel for fixing this!