Corrected external repositories in build.md
Corrected external repositories in build.md

We moved CkEditor in here, so this PR updates our build docs accordingly.
valadas/Dnn.Platform,mitchelsellers/Dnn.Platform,dnnsoftware/Dnn.Platform,dnnsoftware/Dnn.Platform,mitchelsellers/Dnn.Platform,mitchelsellers/Dnn.Platform,dnnsoftware/Dnn.Platform,mitchelsellers/Dnn.Platform,valadas/Dnn.Platform,valadas/Dnn.Platform,valadas/Dnn.Platform,mitchelsellers/Dnn.Platform,valadas/Dnn.Platform,dnnsoftware/Dnn.Platform,dnnsoftware/Dnn.Platform