Document that the Primitives.group does not take a primitive

[axelson]

Document that the Primitives.group does not take a primitive

Motivation and Context

I didn't understand at first why I was getting a function clause error when passing in a primitive even though the documentation says that it is accepted. So I'm fixing the documentation.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.
• The PR title is no longer than 64 characters.

[axelson]

Related: #27

[codecov]

Codecov Report

Merging #168 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #168   +/-   ##
=======================================
  Coverage   85.87%   85.87%           
=======================================
  Files          87       87           
  Lines        2343     2343           
=======================================
  Hits         2012     2012           
  Misses        331      331

Impacted Files	Coverage Δ
lib/scenic/primitives.ex	93.15% <100%> (ø)	⬆️

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