docs(workflow): demonstrate Select in the purchase-order example

[gomitrah]

Description

Updates the Go example in the "External system interaction" pattern
(workflow-patterns.md) to demonstrate the new WorkflowContext.Select
combinator added in dapr/durabletask-go #131,
per this review comment
asking for Go workflow docs examples of the new function.

The Go PurchaseOrderWorkflow example previously relied on
WaitForExternalEvent's built-in timeout and returned a vague
"error/cancelled", err on timeout. This brings it to parity with the
Python (when_any) and JavaScript (whenAny) versions of the same
example on this page, which already explicitly race the approval event
against a separate timer and return a clean "Cancelled" string.

While rewriting this function, also fixed two calls to
workflow.ActivityInput(order) -- a function that doesn't exist in the
real durabletask-go workflow package -- to the correct
workflow.WithActivityInput(order). These two calls sit directly
inside the function being rewritten for the Select example; the same
incorrect name still appears in ~13 other places across this repo
(workflow-patterns.md, workflow-features-concepts.md,
howto-author-workflow.md, and sdkdocs/go/.../go-client/_index.md),
which is out of scope for this PR and left for a follow-up.

Issue reference

No dapr/docs issue exists for this. Responds to a review request on
dapr/durabletask-go#131.