Issue on page /tutorials/means.html

I should do some more work before posting an issue I guess, but I'm not sure when that's going to happen :-)

While reading the tutorial on Fitting a Mean Function](https://tinygp.readthedocs.io/en/stable/tutorials/means.html) I see a few places focusing on vmap and making the mean a function of a single input coordinate. For example:

we must define our mean function to operate on a single input coordinate. What that means is that we don’t need to worry about broadcasting and stuff within our mean function: tinygp will do all the necessary vmap-ing. More explicitly, if we try to call our mean_function on a vector of inputs

Before playing around with the code I find this confusing. I get that if we have a several data points x_i with i=1...N that we would want to vmap that. But if each x_i is a D-dim vector, then I would expect the mean function to depend on all D components of x_i. The way I interpret "single input coordinate" is one of the D components of a single input point x_i. I wonder if what is meant here is "single input data point". With the D=1 example it's hard to tell.

PS: I love the point here

It is quite common in the GP literature to (“without lack of generality”) set the mean of the process to zero and call it a day.

suggest

It is quite common in the GP literature to set the mean of the process to zero (“without lack of generality”) and call it a day.

Open tutorials/means.html and read the section discussing vmap and mean functions for a single input coordinate. Check whether the wording describes one input data point rather than one coordinate, and review the suggested reordering of the “without lack of generality” sentence. Done means the tutorial is unambiguous for multidimensional inputs and the grammar is improved.