message: update dotnet documentation path
  Apparently, this page is also moved and now not maintained "https://hyperledger.github.io/indy-sdk/wrappers/dotnet/docs/index.html". However, I located the documentation link in within the repository and hence updated it here. Please verify and approve/replace with the new link. Thank you.
diff: @@ -9,7 +9,7 @@ Pull requests welcome!
  ### Documentation
  -Documentation for the .NET wrapper is available [here](http://hyperledger.github.io/indy-sdk/wrappers/dotnet/docs/index.html).
  +Documentation for the .NET wrapper is available [here](https://github.com/hyperledger/indy-sdk/blob/master/wrappers/dotnet/docs/index.html).
  ### How to build
  