fix `` code in docstrings

[Wikunia]

How did you address these issues with this PR? What methods did you use?
Twice `` was in the docstrings to start an inline code "block" which I changed to `

[codecov]

Codecov Report

Merging #411 (a4d4892) into master (2f9e047) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   96.25%   96.31%   +0.05%     
==========================================
  Files          35       35              
  Lines        1521     1545      +24     
==========================================
+ Hits         1464     1488      +24     
  Misses         57       57              

Impacted Files	Coverage Δ
src/shorthands/JCircle.jl	100.00% <ø> (ø)
src/shorthands/JEllipse.jl	100.00% <ø> (ø)
src/backgrounds.jl	100.00% <0.00%> (ø)

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