khelp document update

[smart . luck6184]

Hello, I was reading "Chapter 2. Installing KDE Plasma Desktop and KDE 
Applications" and noticed that the Windows and Mac section links lead to a dead 
page. mac.kde.org should be https://community.kde.org/Mac and windows.kde.org 
should be https://community.kde.org/Windows just wanted to send in the 
correction.