internal/readme: Use benchmarks submodule
internal/readme: Use benchmarks submodule

The benchmarks package has been placed inside its own module so that its
dependencies are not considered Zap's dependencies.

This changes the README update script to run the benchmark inside the
benchmark module's directory since we can no longer invoke it from Zap's
top-level directory.

uber-common/zap,uber-go/zap,uber-go/zap