Fix 'platform_name' metadata in ACSPO and CLAVR-x readers

[katherinekolman]

This is a minor fix that changes a variable name in two of the readers to make it consistent across all of them.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[djhoese]

When this passes I'll merge it to master and cherry-pick it to the maint/0.9 branch and make a new 0.9 release.

[coveralls]

Coverage remained the same at 69.273% when pulling 6000729 on katherinekolman:master into c799d3a on pytroll:master.

[codecov]

Codecov Report

Merging #402 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   69.27%   69.27%           
=======================================
  Files         123      123           
  Lines       15833    15833           
=======================================
  Hits        10968    10968           
  Misses       4865     4865

Impacted Files	Coverage Δ
satpy/readers/acspo.py	85.71% <ø> (ø)	⬆️
satpy/readers/clavrx.py	84.14% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c799d3a...6000729. Read the comment docs.