gettingstarted: Update docs to use head instead of a branch

We always work on HEAD as our next release, so elide the branch designation.

Change-Id: I9283140e7f60bdbe6db66f5d5e5fcb1e8f910fdc
diff --git a/GettingStarted.md b/GettingStarted.md
index 61be9e6..6c46ded 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -34,7 +34,7 @@
 current release branch.
 
 ```
-repo init -u https://coral.googlesource.com/manifest -b release-day
+repo init -u https://coral.googlesource.com/manifest
 repo sync -j$(nproc)
 ```