Update docs in hello world tutorial (#19)

Updaet docs in hello world tutorial

Motivation:

The hello world tutorial tells you to grpc/grpc-swift at a particular
commit. This works fine but the code has moved to a new home now.

Modifications:

- Update link and remove branch

Result:

Docs are up-to-date

Author: glbrntt

Sources/GRPCCore/Documentation.docc/Tutorials/Hello-World/Hello-World.tutorial

@@ -18,7 +18,7 @@
     repository by running the following command in a terminal:
 
     ```console
-    git clone --branch 2.0.0 https://github.com/grpc/grpc-swift
+    git clone https://github.com/grpc/grpc-swift-2
     ```
 
     You then need to change directory to the `Examples/hello-world` directory of the cloned