sig-architecture: update code generation makefile hints

[mattcary]

The sig-architecture api developer page has some stale references to the old generated_files make rule which no longer exists. This PR changes it to the current make update rule.

I've tested it by deleting the referenced files and checking that they get regenerated. But I don't claim to understand these make rules so please double-check.

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This pull-request has been approved by: mattcary
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[palnabarun]

/cc @dims for a review from sig-arch side

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/cc @dims for a review from sig-arch side

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