update installation instructions to request version >5 (#6752)

Author: error9098x

docs/source/installation.md

@@ -6,7 +6,7 @@ We recommend using [Anaconda](https://www.anaconda.com/) (or [Miniforge](https:/
 Once you have installed one of the above, PyMC can be installed into a new conda environment as follows:
 
 ```console
-conda create -c conda-forge -n pymc_env "pymc>=4"
+conda create -c conda-forge -n pymc_env "pymc>=5"
 conda activate pymc_env
 ```
 If you like, replace the name `pymc_env` with whatever environment name you prefer.