[CI-Examples] Increase enclave size to 512MB for Nginx

On some systems (namely CentOS/RHEL), the previous enclave size of 256MB
was barely enough for Nginx worker processes. Intermittently, these
worker processes failed with `Out-of-memory in library OS` error.
Increasing the size to 512MB fixes this error.

Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>

CI-Examples/nginx/nginx.manifest.template

@@ -36,7 +36,7 @@ fs.mount.cwd.uri = "file:{{ install_dir }}"
 
 sgx.debug = true
 sgx.nonpie_binary = true
-sgx.enclave_size = "256M"
+sgx.enclave_size = "512M"
 sgx.thread_num = 4
 
 # Nginx benefits from Exitless. Uncomment the below line to use it.