Update the JupyterLab version in README

Author: krassowski

README.md

@@ -99,9 +99,9 @@ Use of a python `virtualenv` or a conda env is also recommended.
 1. install JupyterLab
 
    ```bash
-   conda install -c conda-forge 'jupyterlab>=2,<2.1.0a0'
+   conda install -c conda-forge 'jupyterlab>=2.1,<3.0.0a0'
    # or
-   pip install 'jupyterlab>=2,<2.1.0a0'
+   pip install 'jupyterlab>=2.1,<3.0.0a0'
    ```
 
 1. install the server extension: