Fix incorrect parameter name for Google system instruction

[felixarntz]

Fixes #177

See https://ai.google.dev/api/generate-content#request-body: The correct field name is systemInstruction.

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[JasonTheAdams]

@felixarntz Am I correct in looking at the instructions you linked to that systemInstruction is actually meant to be a Content object (as opposed to a string)? It says it only supports text, but it sounds like it may mean that as parts and such. But maybe I'm misreading that.

[felixarntz]

@JasonTheAdams Yikes! You're right. Fixed in a27a8f2

Tested it via --systemInstruction in our cli.php and confirmed that it works.