Fix typo in variable names in resample documentation

[djhoese]

The variable says "global_scene" but is actually "scn". Not sure how/when this happened, but this PR fixes it.

• Closes #xxxx
• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage remained the same at 90.065% when pulling 0fe675b on djhoese:doc-resample-typo into e0773c4 on pytroll:master.

[codecov]

Codecov Report

Merging #1317 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1317   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files         220      220           
  Lines       32420    32420           
=======================================
  Hits        29199    29199           
  Misses       3221     3221           

Impacted Files	Coverage Δ
satpy/resample.py	90.77% <ø> (ø)

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[pnuu]

LGTM