Fixed minor typo

turotial -> tutorial
Merge branch 'andydavies-master'
Remove -Wno-unused-parameter (and add _U_ macro)
Fix -Werror=unused-parameter using _U_ macro
Merge branch 'Wunused-parameter' of https://github.com/alagoutte/nghttp2 into alagoutte-Wunused-parameter
Merge branch 'alagoutte-Wunused-parameter'
Add Wdocumentation Clang Flag