fix setting of http.Handler in the example server

[marten-seemann]

... when run with the -tcp flag.

[codecov]

Codecov Report

Merging #2900 (eb53fdf) into master (69158cf) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2900      +/-   ##
==========================================
- Coverage   85.77%   85.76%   -0.01%     
==========================================
  Files         133      133              
  Lines        9187     9181       -6     
==========================================
- Hits         7880     7874       -6     
  Misses        959      959              
  Partials      348      348              

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)
internal/flowcontrol/connection_flow_controller.go	100.00% <100.00%> (ø)
internal/flowcontrol/stream_flow_controller.go	100.00% <100.00%> (ø)

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