Change @time to @btime in need for speed

[jlperla]

And put in a tiny example paragraph at the top for BenchmarkTools.jl

Make sure that we tell people to always use the $ interpolation.

[Nosferican]

For benchmarking @btime or @benchmark will do. I have been using the $ interpolation, but would be good to check the intro section has an example (with and without).

[jlperla]

For sure. The issue with the interpolatio is that it is very subtle and easy to forget. Maybe we can get around to helping with JuliaCI/BenchmarkTools.jl#118 (comment) after the v1.0 launch of the lecture notes.

[arnavs]

Done at bf9ab32.