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

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: I5fef990f17333507c2e9d1ac73ead5941520764b
diff --git a/mbed-tls-pr-head.yaml b/mbed-tls-pr-head.yaml
index a55d347..be02109 100644
--- a/mbed-tls-pr-head.yaml
+++ b/mbed-tls-pr-head.yaml
@@ -21,7 +21,7 @@
                       <repositoryUrl>https://github.com/Mbed-TLS/mbedtls-test.git</repositoryUrl>
                     </scm>
                   </retriever>
-                  <defaultVersion>master</defaultVersion>
+                  <defaultVersion>main</defaultVersion>
                   <implicit>true</implicit>
                   <allowVersionOverride>true</allowVersionOverride>
                   <includeInChangesets>true</includeInChangesets>