Correct name of LTS branch to mbedtls-2.28

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: I7dd159c58b8b9cfff97292c2a6dbe75c48846143
diff --git a/mbed-tls-nightly-tests.yaml b/mbed-tls-nightly-tests.yaml
index 8b101b9..c3f5524 100644
--- a/mbed-tls-nightly-tests.yaml
+++ b/mbed-tls-nightly-tests.yaml
@@ -132,4 +132,4 @@
         properties-content: GIT_CREDENTIALS_ID=mbedtls-github-token
     triggers:
     - parameterized-timer:
-        cron: "H(0-29) 20 * * * %MBED_TLS_BRANCH=development \nH(0-29) 20 * * * %MBED_TLS_BRANCH=mbedtls-2.18"
+        cron: "H(0-29) 20 * * * %MBED_TLS_BRANCH=development \nH(0-29) 20 * * * %MBED_TLS_BRANCH=mbedtls-2.28"