Updates to SymbolSource and Contributing-to-MyGet-Documentation

On **SymbolSource.markdown**, under "Pushing a package to myget.org:"

changed this:

```nuget.exe push <package-file> -Source https://nuget.gw.symbolsource.org/MyGet/<feed-name>```

to this:

```nuget.exe push <package-file> -Source https://www.myget.org/F/<feed-name>```

On **Contributing-to-MyGet-Documentation.markdown**, the "reading is here" link was broken, so I fixed it