REGTESTS: ssl: fix grep invocation to use extended regex in ssl_generate_certificate.vtc

in 2f2a2884b7464ccb56469cb94d8a1ae4015a8cb6 grep should have use regex flag -E, but flag
was lost by mistake
diff --git a/reg-tests/ssl/ssl_generate_certificate.vtc b/reg-tests/ssl/ssl_generate_certificate.vtc
index b3d9201..96549df 100644
--- a/reg-tests/ssl/ssl_generate_certificate.vtc
+++ b/reg-tests/ssl/ssl_generate_certificate.vtc
@@ -164,5 +164,5 @@
 }
 
 shell {
-    echo "Q" | openssl s_client -unix "${tmpdir}/ssl_P-384.sock" -servername server.ecdsa.com 2>/dev/null| grep "Temp Key: ECDH,.+, 384 bits"
+    echo "Q" | openssl s_client -unix "${tmpdir}/ssl_P-384.sock" -servername server.ecdsa.com 2>/dev/null| grep -E "Temp Key: ECDH,.+, 384 bits"
 }