Ransomware targeted files

[mssalvatore]

What does this PR do?

Describe ransomware extensions as "targeted", rather than valid (which implies invalid). Make language consistent between the documentation and the code.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by running Hugo and unit tests

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Merging #1284 (ebab7be) into develop (3fb8c06) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1284   +/-   ##
========================================
  Coverage    30.68%   30.68%           
========================================
  Files          449      449           
  Lines        13466    13466           
========================================
  Hits          4132     4132           
  Misses        9334     9334           

Impacted Files	Coverage Δ
.../infection_monkey/ransomware/ransomware_payload.py	95.45% <100.00%> (ø)
...tion_monkey/ransomware/targeted_file_extensions.py	100.00% <100.00%> (ø)

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