Update metadata-filtering.mdx

[freetonik]

I was a bit confused with the example { "$ne": true }, it seemed like one should put true specifically, and then provide the actual value elsewhere. Unless I misunderstood again, perhaps this small change would make it clearer how the operators work.

Summary

Vectorize → Reference → Metadata filtering article.

• [x] 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• [ ] If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.