Fix VIIRS 'night_fog' RGB composite recipe to use M12 instead of M14

[adybbroe]

The night_fog RGB composite recipe was wrong, seems like a cut and paste error. The M14 band was used (as for the fog RGB) whwre M12 should have been used

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #719 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   80.01%   80.02%   +<.01%     
==========================================
  Files         147      147              
  Lines       21498    21498              
==========================================
+ Hits        17202    17203       +1     
+ Misses       4296     4295       -1

Impacted Files	Coverage Δ
satpy/scene.py	89.89% <0%> (+0.18%)	⬆️

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[coveralls]

Coverage increased (+0.005%) to 80.021% when pulling eab5f31 on adybbroe:bugfix-nightfog-rgb-viirs into 7f86b2b on pytroll:master.