Fixed the typo in the serializer

[cmodijk]

See https://github.com/SimpleBus/Serialization/issues/4

[Nyholm]

Don't we need more changes than the docs?

[cmodijk]

@Nyholm No I checked the MessageInEnvelopSerializer is no where to be found.

[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #6   +/-   ##
=======================================
  Coverage   78.26%   78.26%           
=======================================
  Files           2        2           
  Lines          23       23           
=======================================
  Hits           18       18           
  Misses          5        5

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