Fix typo at UNKNOWN word

[leocavalcante]

Closes #5236

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (3ace5d6) 72.96% compared to head (1a0d8a2) 72.93%.

Files	Patch %	Lines
src/protocol/http2.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5237      +/-   ##
==========================================
- Coverage   72.96%   72.93%   -0.04%     
==========================================
  Files          69       69              
  Lines       14858    14858              
==========================================
- Hits        10841    10836       -5     
- Misses       4017     4022       +5     

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