doc: CLI_ABI.md: fix exitcodes.go link
doc: CLI_ABI.md: fix exitcodes.go link

Needs a leading "..".

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs