Fix documentation for get_position on Layer

[EnzioKam]

PR Checklist

If you are contributing to Javis.jl, please make sure you are able to check off each item on this list:

• Did I update CHANGELOG.md with whatever changes/features I added with this PR?
• Did I make sure to only change the part of the file where I introduced a new change/feature?
• Did I cover all corner cases to be close to 100% test coverage (if applicable)?
• Did I properly add Javis dependencies to the Project.toml + set an upper bound of the dependency (if applicable)?
• Did I properly add test dependencies to the test directory (if applicable)?
• Did I check relevant tutorials that may be affected by changes in this PR?
• Did I clearly articulate why this PR was made the way it was and how it was made?

Link to relevant issue(s)

How did you address these issues with this PR? What methods did you use?
The type annotation in documentation for get_position on Layers here is a duplicate of the one for Object here.

This PR just changes 1 line to show the correct type annotation in the documentation.

[codecov]

Codecov Report

Merging #401 (d484a19) into master (05a87b9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #401   +/-   ##
=======================================
  Coverage   96.25%   96.25%           
=======================================
  Files          35       35           
  Lines        1521     1521           
=======================================
  Hits         1464     1464           
  Misses         57       57           

Impacted Files	Coverage Δ
src/layers.jl	84.37% <ø> (ø)

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[Wikunia]

Thanks