Fix comment when build with -Wdocumentation and Clang

./includes/nghttp2/nghttp2.h:70:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:1536:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:2724:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:2850:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration