Fix security audit date format (#3428)

Re-apply fix applied in #3418 that was accidentally reverted in #3424.

Signed-off-by: Cintia Sanchez Garcia <cynthiasg@icloud.com>

landscape.yml

@@ -9310,7 +9310,7 @@ landscape:
               summary_intro_url: ''
               clomonitor_name: jaeger
               audits:
-                - date: 2019--05-04
+                - date: '2019-05-04'
                   type: security
                   url: https://github.com/jaegertracing/security-audits/blob/master/SECURITY_AUDIT_2019_05_04.pdf
                   vendor: Cure53