Update installation instructions

Use Mambaforge rather than installing miniconda and mamba in two steps

Author: davewhipp

README.md

@@ -3,19 +3,16 @@
 ## Starting steps
 This briefly describes how to install a Python virtual environment for the Geo-Python course.
 
-1. Install miniconda from https://docs.conda.io/en/latest/miniconda.html
-2. Install mamba
-    ```bash
-    conda install mamba -n base -c conda-forge
-    ```
-3. Clone the course Python environments from GitHub
+1. Install mambaforge from [https://docs.conda.io/en/latest/miniconda.html](https://github.com/conda-forge/miniforge#mambaforge).
+2. Clone the course Python environments from GitHub
     ```bash
     git clone https://github.com/geo-python/python-environments.git
     ```
-4. Change into the working directory with the cloned environment files
+3. Change into the working directory with the cloned environment files
     ```bash
     cd python-environments
     ```
+
 ## Creating the environment for students
 1. Create the python environment using mamba
     ```bash
@@ -26,6 +23,7 @@ This briefly describes how to install a Python virtual environment for the Geo-P
     conda activate geo-python-student
     sh postBuild
     ```
+
 ## Creating the environment for teachers
 1. Create the python environment using mamba
     ```bash