README: update arbiter from rsc@ to austin@

Change-Id: I4fc5ffa54a5a10fe76d76193ead6be0c271b2110
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/756500
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Austin Clements <austin@google.com>

README.md

diff --git a/README.md b/README.md
index c0da150..1d2278a 100644
--- a/README.md
+++ b/README.md
@@ -335,7 +335,7 @@
 
 If consensus among the architects cannot be reached,
 which is even more unusual,
-the arbiter (currently [rsc@](mailto:rsc@golang.org))
+the arbiter (currently [austin@](mailto:austin@google.com))
 reviews the discussion and decides the next step,
 documenting the decision and its rationale on the issue.