doc: fix escaping of backslash

[aviatesk]

in doc string we need more escape characters

[codecov-commenter]

Codecov Report

Merging #58 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #58   +/-   ##
=======================================
  Coverage   65.45%   65.45%           
=======================================
  Files           6        6           
  Lines         359      359           
=======================================
  Hits          235      235           
  Misses        124      124           

Impacted Files	Coverage Δ
src/printing.jl	95.83% <ø> (ø)

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[jlumpe]

Thanks!