Update references to old Github organisation

Replace references to ARMmbed organisation with the new
org, Mbed-TLS, following project migration.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/compat.sh b/tests/compat.sh
index 42b9cdb..a03407c 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -446,7 +446,7 @@
 #
 # NOTE: for some reason RSA-PSK doesn't work with OpenSSL,
 # so RSA-PSK ciphersuites need to go in other sections, see
-# https://github.com/ARMmbed/mbedtls/issues/1419
+# https://github.com/Mbed-TLS/mbedtls/issues/1419
 #
 # ChachaPoly suites are here rather than in "common", as they were added in
 # GnuTLS in 3.5.0 and the CI only has 3.4.x so far.