Fix StreamServerInterceptor doc example

[Julio-Guerra]

Fix the example showing how to store a value into a stream context:

• get the stream context out of the stream object.
• pass the newly created wrapped stream to the handler.

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[codecov-io]

Codecov Report

Merging #195 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   73.09%   73.09%           
=======================================
  Files          36       36           
  Lines        1349     1349           
=======================================
  Hits          986      986           
  Misses        314      314           
  Partials       49       49

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