Fix typo in header.md

[codecov-commenter]

Codecov Report

Merging #393 into master will increase coverage by 2.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   72.05%   74.23%   +2.18%     
==========================================
  Files          25       25              
  Lines        1249     1347      +98     
==========================================
+ Hits          900     1000     +100     
+ Misses        349      347       -2     

Flag	Coverage Δ
#unittests	74.23% <ø> (+2.18%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/cache.jl	100.00% <0.00%> (ø)
src/types.jl	100.00% <0.00%> (ø)
src/writer/writer.jl	100.00% <0.00%> (ø)
src/reader/notebook.jl	100.00% <0.00%> (ø)
src/reader/markdown.jl	97.95% <0.00%> (+0.13%)	:arrow_up:
src/converter.jl	98.59% <0.00%> (+0.15%)	:arrow_up:
src/reader/script.jl	87.75% <0.00%> (+0.25%)	:arrow_up:
src/rendering/pandocformats.jl	85.18% <0.00%> (+0.56%)	:arrow_up:
src/reader/reader.jl	90.38% <0.00%> (+0.80%)	:arrow_up:
src/rendering/common.jl	88.23% <0.00%> (+0.89%)	:arrow_up:
... and 10 more

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[aviatesk]

thx !