Update instructions to build release-day branch

Change-Id: Ia07fd668795618d2ec6551fc4fd52aa0f78ac72c
Signed-off-by: Leonid Lobachev <leonidl@google.com>

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index a24433f..7d411e8 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -32,7 +32,7 @@
 current release branch.
 
 ```
-repo init -u https://coral.googlesource.com/manifest -b release-chef
+repo init -u https://coral.googlesource.com/manifest -b release-day
 repo sync -j$(nproc)
 ```