Fix diagram image size.
Fix diagram image size.

Switched line from markup to html so that we can append the "img-responsive" class.
wildebeestdev/grpc.github.io,wildebeestdev/grpc.github.io,wildebeestdev/grpc.github.io,avx2/avx2.github.io,avx2/avx2.github.io,avx2/avx2.github.io