ssl-opt.sh: fix typo in mtu settings

diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 5483734..f02d2ec 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -5964,7 +5964,7 @@
              crt_file=data_files/server7_int-ca.crt \
              key_file=data_files/server7.key \
              hs_timeout=2500-60000 \
-             mtu=512" \
+             mtu=1024" \
             "$P_CLI dtls=1 debug_level=2 \
              crt_file=data_files/server8_int-ca2.crt \
              key_file=data_files/server8.key \