Fix typo in Must docs

[craigpastro]

Whoops. I introduced this typo. Sorry about that.

[abhinav]

Thanks!

[codecov]

Codecov Report

Merging #1108 (9082137) into master (e06e09a) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
==========================================
+ Coverage   98.27%   98.36%   +0.09%     
==========================================
  Files          49       49              
  Lines        2145     2145              
==========================================
+ Hits         2108     2110       +2     
+ Misses         28       27       -1     
+ Partials        9        8       -1     

Impacted Files	Coverage Δ
logger.go	96.61% <ø> (ø)
zapcore/sampler.go	100.00% <0.00%> (+3.77%)	⬆️

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