Fix Issue 1340 - Add language to uninstall Windows

[prcutler]

This fixes Mu issue 1340: mu-editor/mu#1340

This changes the paragraph in the Windows install page about uninstalling an older version of Mu and changes it to:

**If you have a version of Mu older than version 1.1 already installed, we recommend that you
uninstall it first.  If you do not uninstall first, you will have two installations of Mu due to 
packaging changes with Mu versions 1.1 and newer.**

Sorry about the commit revert, I had an issue with my repo, this PR should be just the one commit. If I need to change anything, please let me know.