Documentation - gentoo section trivial tweak required

[francoisjacques]

The Gentoo section should read:

sudo emerge dev-libs/boost dev-libs/libsodium sys-libs/ncurses \
	dev-libs/protobuf dev-util/cmake dev-cpp/gflags

Instead of dev-cpp/gflag.

EDIT: URL to package - https://packages.gentoo.org/packages/dev-cpp/gflags