fix: typo in argument

[Jason Schmidt]

Describe the bug
This is what we show when the user requests help:

Modern Application Reference Architecture (MARA) Runner

USAGE:
    main.py [FLAGS] [OPERATION]

FLAGS:
    -d, --debug        Enable debug output on all of the commands executed
    -b, --banner-type= Banner type to indicate which project is being executed (e.g. fabulous, boring, log)
    -h, --help         Prints help information
    -s, --stack        Specifies the Pulumi stack to use
    -p, --provider=    Specifies the provider used (e.g. linode, aws, do)

OPERATIONS:
    down/destroy    Destroys all provisioned infrastructure
    list-providers  Lists all of the supported providers
    refresh         Refreshes the Pulumi state of all provisioned infrastructure
    show-execution  Displays the execution order of the Pulumi projects used to provision
    up              Provisions all configured infrastructure
    validate        Validates that the environment and configuration is correct

The -s/--stack argument requires a value to be passed, so we should update the help text to include an = sign.

To Reproduce
Steps to reproduce the behavior:

1. Display help.

Expected behavior
Should show the correct help, with the right values.

Your environment

• n/a

Additional context
None

[Jason Schmidt]

Closed by #210