Rename mbedtls-test branch "master" to "main"

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: I5fef990f17333507c2e9d1ac73ead5941520764b
diff --git a/mbed-tls-nightly-tests.yaml b/mbed-tls-nightly-tests.yaml
index a86becf..28c2f77 100644
--- a/mbed-tls-nightly-tests.yaml
+++ b/mbed-tls-nightly-tests.yaml
@@ -112,7 +112,7 @@
       scm:
       - git:
           branches:
-          - 'master'
+          - main
           url: https://github.com/Mbed-TLS/mbedtls-test.git
       script-path: vars/mbedtls-release-Jenkinsfile
     project-type: pipeline