Update waf-managed-rulesets.mdx

[cf-scott]

"version" is a Read-Only field, and throws an error if included in a resource block.

https://registry.terraform.io/providers/cloudflare/cloudflare/4.48.0/docs/resources/ruleset

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Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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