Fix typo in system-environment-variables.mdx

[sandstreampop]

Summary

My change fixes a typo (missing < at the beginning of a Type element) on the https://developers.cloudflare.com/workers/wrangler/system-environment-variables page.

Screenshots (optional)

The typo is underlined in red.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

[WalshyDev]

Thank you!

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