repository.yml: Remove invalid stability "rc1"

Newt only allows the following stability strings in a repo version:

    VERSION_STABILITY_NONE   = ""
    VERSION_STABILITY_STABLE = "stable"
    VERSION_STABILITY_DEV    = "dev"
    VERSION_STABILITY_LATEST = "latest"
    VERSION_STABILITY_COMMIT = "commit"

An attempt to update a project that uses mcuboot would choke due to the
invalid stability string "rc1":

    Error: Unknown stability (rc1) in version 1.3.0-rc1

Signed-off-by: Christopher Collins <ccollins@apache.org>
diff --git a/repository.yml b/repository.yml
index 7bf185c..636c621 100644
--- a/repository.yml
+++ b/repository.yml
@@ -24,7 +24,7 @@
     "1.0.0": "v1.0.0"
     "1.1.0": "v1.1.0"
     "1.2.0": "v1.2.0"
-    "1.3.0-rc1": "v1.3.0-rc1"
+    "1.3.0": "v1.3.0-rc1"
 
     "0-dev": "0.0.0"        # master
     "0-latest": "1.2.0"     # latest stable release