Update the monorepo URL in a lld's document.

llvm-svn: 313396
diff --git a/lld/docs/index.rst b/lld/docs/index.rst
index fbdade7..272b3c1 100644
--- a/lld/docs/index.rst
+++ b/lld/docs/index.rst
@@ -110,7 +110,7 @@
 
 .. code-block:: console
 
-  $ git clone https://github.com/llvm-project/llvm-project/
+  $ git clone https://github.com/llvm-project/llvm-project-20170507 llvm-project
   $ mkdir build
   $ cd build
   $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm