fix: Instruct users to run from 'lab' environment (#114)

* To run notebooks using Jupyter Lab users should install from and run
  in the 'lab' environment which extends the default environment with
  all the core dependencies to include 'notebook' and 'jupyterlab'.

Author: matthewfeickert

book/introduction.md

@@ -167,13 +167,13 @@ To get all the dependencies needed for this tutorial first clone the repository
 then simply run
 
 ```
-pixi install
+pixi install --environment lab
 ```
 
 or to also start running the example notebooks run
 
 ```
-pixi run start
+pixi run --environment lab lab
 ```
 
 #### Using `pip`