Documentation error

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#### Overview of the Issue

There seems to be an error in the [shell-local processor documentation](https://developer.hashicorp.com/packer/docs/provisioners/shell-local#execute_command) - the default is probably actually `["/bin/sh", "-c", "{{.Vars}} {{.Script}}"]`, like what is shown in the [shell-local _post-processor_ documentation](https://developer.hashicorp.com/packer/docs/post-processors/shell-local#execute_command), though I have not verified that.

I have verified that it isn't `["/bin/sh", "-c", "{{.Vars}}", "{{.Script}}"]`, as plugging that into `execute_command` causes the script to not run.

#### Proposed fix
```diff
-["/bin/sh", "-c", "{{.Vars}}", "{{.Script}}"]
+["/bin/sh", "-c", "{{.Vars}} {{.Script}}"]
```

Start with the shell-local processor documentation's execute_command section and compare it with the linked shell-local post-processor documentation. Confirm the actual default before editing the documented command, then verify that the processor documentation matches the implementation and the post-processor example.

Written by the indexing model from the issue text.