Correct the README link in Unix install docs (#1245)

diff --git a/Doc/using/unix.rst b/Doc/using/unix.rst
index 97f0a49..604688c 100644
--- a/Doc/using/unix.rst
+++ b/Doc/using/unix.rst
@@ -77,7 +77,7 @@
    make install
 
 invocations. Configuration options and caveats for specific Unix platforms are
-extensively documented in the :source:`README` file in the root of the Python
+extensively documented in the :source:`README.rst` file in the root of the Python
 source tree.
 
 .. warning::