Fix path for download_prebuilt.sh (#635)

Remove the unnecessary '/'

Author: PumpkinSeed

download_prebuilt.sh

@@ -37,7 +37,7 @@ optlib_name=optimized_libs_update_2.12.tar.gz
 ae_file_name=prebuilt_ae_2.12.tar.gz
 binutils_file_name=as.ld.objdump.gold.r3.tar.gz
 checksum_file=SHA256SUM_prebuilt_2.12.cfg
-server_url_path=https://download.01.org/intel-sgx/sgx-linux/2.12/
+server_url_path=https://download.01.org/intel-sgx/sgx-linux/2.12
 server_optlib_url=$server_url_path/$optlib_name
 server_ae_url=$server_url_path/$ae_file_name
 server_binutils_url=$server_url_path/$binutils_file_name