Fix FY-4 naming to follow WMO Oscar naming

[adybbroe]

Signed-off-by: Adam Dybbroe Adam.Dybbroe@smhi.se

Fixes the platform_name in the FY-4 AGRI reader, so the naming follows the WMO/Oscar standard.

• Tests passed
• Passes flake8 satpy

[djhoese]

@adybbroe If you have time, could you merge this with master so the tests rerun?

[mraspaud]

LGTM

[mraspaud]

@adybbroe can you try merging with master ?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@34e4045). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #871   +/-   ##
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  Coverage          ?   85.15%           
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  Files             ?      174           
  Lines             ?    25502           
  Branches          ?        0           
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  Hits              ?    21717           
  Misses            ?     3785           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/readers/agri_l1.py	99.04% <100%> (ø)

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