docs(getting-started): fix tutorial BPMN workflow

[pinkertonpg]

Description

Per error received when deploying the example workflow, expressions must start with '=':

Error: rpc error: code = InvalidArgument desc = Command rejected with code 'CREATE': Expected to deploy new resources, but encountered the following errors:
'order-process.bpmn': - Element: Message_155nrcd > extensionElements > subscription
    - ERROR: Expected expression but found static value 'orderId'. An expression must start with '=' (e.g. '=orderId').
- Element: SequenceFlow_1girnrf > conditionExpression
    - ERROR: Expected expression but found static value 'orderValue>=100'. An expression must start with '=' (e.g. '=orderValue>=100').

Related issues

Closes #4421

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