Since ESW 500 Series firmware version 2.1.16 requires boot version 22.214.171.124 for 8-port ESW models and boot version 126.96.36.199 for 24-port and 48-port models, you may need to manually install or upgrade the boot code to ensure that the appropriate versions are used.
To download firmware and boot code files for ESW switches, go to www.cisco.com/go/esw500help, click the Resources tab, then click the Firmware link. A Cisco.com login is required.
Unable to find boot code at the listed URL.
And remember to use the Browser interface to the switch to install new bootcode. CCA does not do boot code.
I followed the link you mentioned, selected firmware and rummaged around and found the bootcode.
see my screenshot below.