Updated the file name for the FY-3F Mersi-3 RSRs

[adybbroe]

The purpose of this PR is to handle the fact that Pyspectral expects the RSR filename for Mersi-3 FY-3F to be rsr_mersi3_FY-3F.h5 rather than rsr_mersi-3_FY-3F.h5

• Closes #xxxx
• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (102a19f) to head (29663bc).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   89.82%   89.82%           
=======================================
  Files          23       23           
  Lines        2535     2535           
=======================================
  Hits         2277     2277           
  Misses        258      258           

Flag	Coverage Δ
unittests	89.82% <100.00%> (ø)

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[simonrp84]

LGTM!