Update Version of MCUBOOT

Preparation for TF-M V1.3 Release dry run.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I6deaf0c6f67837592d2a3c1d53ca63925403c4fa
diff --git a/tf-m-build-docs-nightly.yaml b/tf-m-build-docs-nightly.yaml
index 93046dc..240c98c 100644
--- a/tf-m-build-docs-nightly.yaml
+++ b/tf-m-build-docs-nightly.yaml
@@ -59,7 +59,7 @@
           default: 'master'
       - string:
           name: 'MCUBOOT_REFSPEC'
-          default: '81d19f017f18f1bc6bea906d96fd45fba3672064'
+          default: 'v1.7.2-rc2'
           description: |
             Specify which MCUboot version to use
             refs/heads/master    : Pick up latest master