Add missing word on docs

[Nipodemos]

It is missing a "is" on one sentence I red. This fix it

[codecov-io]

Codecov Report

Merging #4 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   90.86%   90.75%   -0.12%     
==========================================
  Files          27       27              
  Lines         898      898              
==========================================
- Hits          816      815       -1     
- Misses         82       83       +1

Impacted Files	Coverage Δ
hive/lib/src/binary/binary_writer_impl.dart	93.89% <0%> (-0.77%)	⬇️

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[simc]

Thank you!