docs(exec-source): README examples pass a multi-word --command without --shell and fail to start

[rrrodzilla]

What is wrong

Three examples in primitives/exec-source/README.md pass a multi-word command without --shell:

exec-source --command "ls -la"
exec-source --command "df -h" --interval 60000

args = ["--command", "df -h", "--interval", "60000"]

Without --shell, build_command in primitives/exec-source/src/main.rs runs --command as the executable name and takes arguments only from --args. So "df -h" is looked up as a program literally named df -h, the spawn fails, and the source exits with status 1 and this on stderr:

Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

How it was found

Writing a new example topology for the engine docs against the released 0.12.0 binary. args = ["--command", "cat ./message.txt"] copied the README's form and the source died at startup. ["--command", "cat", "--args", "./message.txt"] works.

Fix

Correct the three examples to use --args (or --shell sh), and say in the arguments table that --command is the executable unless --shell is given.

Worth a separate look: the raw Os { code: 2 ... } message does not name the command that was not found.