Corrected command example on SymbolSource.markdown

corrected the URL that followed -Source for the "Pushing a package to myget.org" example under "Quick command cheatsheet"
Update Contributing-to-MyGet-Documentation

fixed the broken link that attempted to point potential contributors to the non-editable version of this very page
Merge pull request #43 from jimmym715/updates

Updates to SymbolSource and Contributing-to-MyGet-Documentation