README: fix invocation of Go routine in example

[gokpm]

Before

The example goroutine provided in the README.md is not called, it's just defined.

After

Call the goroutine

[google-cla]

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[codecov]

Codecov Report

Merging #4019 (73433fb) into master (aab4d4e) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4019      +/-   ##
==========================================
+ Coverage   82.85%   82.89%   +0.04%     
==========================================
  Files         147      147              
  Lines       14779    14779              
==========================================
+ Hits        12244    12250       +6     
+ Misses       2033     2028       -5     
+ Partials      502      501       -1     

see 2 files with indirect coverage changes