Docs: Change `TF_ORGANIZATION` to `TF_CLOUD_ORGANIZATION`

Description

In the documents, TF_ORGANIZATION is referenced as the default vault for organization input (if we don't explicitly set it via with.organization. However, the underlying tool uses TF_CLOUD_ORGANIZATION as the default one. In the example workflows TF_CLOUD_ORGANIZATION is indicated, however, in the actual action input comments, TF_ORGANIZATION is referenced probably due to a typo.

This PR fixes #32.

Testing plan

Try to use the Upload Configuration action by specifying TF_ORGANIZATION and omitting with.organization. It'll result in:

error uploading configuration version to Terraform Cloud: invalid value for organization
panic: reflect: call of reflect.Value.Type on zero Value

goroutine 1 [running]:
reflect.Value.Type({0x0?, 0x0?, 0x[11](https://github.com/PashmakGuru/clusters/actions/runs/7304351620/job/19906395310#step:4:12)0?})
    /usr/local/go/src/reflect/value.go:2453 +0x[12](https://github.com/PashmakGuru/clusters/actions/runs/7304351620/job/19906395310#step:4:13)e
github.com/hashicorp/tfci/internal/command.(*outputMessage).Value(0xc0001cc5d0)
    /src/internal/command/output.go:39 +0xf9
github.com/hashicorp/tfci/internal/command.(*Meta).closeOutput(0xc0004540f0)
    /src/internal/command/meta.go:121 +0x12b
github.com/hashicorp/tfci/internal/command.(*UploadConfigurationCommand).Run(0xc0001cc510, {0xc00001e0d0, 0x3, 0x3})
    /src/internal/command/upload.go:63 +0x61d
github.com/mitchellh/cli.(*CLI).Run(0xc000[14](https://github.com/PashmakGuru/clusters/actions/runs/7304351620/job/19906395310#step:4:15)1a40)
    /go/pkg/mod/github.com/mitchellh/cli@v1.1.5/cli.go:262 +0x5f8
main.realMain()
    /src/main.go:61 +0xf6
main.main()
    /src/main.go:46 +0x1b4

External links

Terraform Cloud CLI Behavior

hashicorp / tfc-workflows-github