Add --recursive flag to git clone command.

To avoid a later git command.  Simpler.

INSTALL.md

@@ -107,9 +107,8 @@ Builds gRPC C and C++ with boringssl.
 Please note that when using Ninja, you'll still need Visual C++ (part of Visual Studio)
 installed to be able to compile the C/C++ sources.
 ```
-> powershell git clone -b ((New-Object System.Net.WebClient).DownloadString(\"https://grpc.io/release\").Trim()) https://github.com/grpc/grpc
+> powershell git clone --recursive -b ((New-Object System.Net.WebClient).DownloadString(\"https://grpc.io/release\").Trim()) https://github.com/grpc/grpc
 > cd grpc
-> git submodule update --init
 > md .build
 > cd .build
 > call "%VS140COMNTOOLS%..\..\VC\vcvarsall.bat" x64