Fix typo in comment for Metadata struct

[calebschoepp]

Just noticed this typo while reading through the docs.

Please let me know if there is anything else I need to do for this PR.

[codecov-commenter]

Codecov Report

Merging #227 into master will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   89.09%   89.65%   +0.55%     
==========================================
  Files          15       15              
  Lines         715      715              
==========================================
+ Hits          637      641       +4     
+ Misses         66       63       -3     
+ Partials       12       11       -1     

Impacted Files	Coverage Δ
shard.go	88.26% <ø> (+1.73%)	⬆️

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[cristaloleg]

Thanks 🎉