fix(plugin-sitemap): update to version 0.8.1

[erezrokah]

Fixes netlify-labs/netlify-plugin-sitemap#71

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• Adding a plugin
• Updating a plugin

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• Read and followed the plugin author guidelines.
• Included all required fields in your entry.
• Tested the plugin locally and on Netlify, using the plugin version stated in your entry.

Test plan
https://app.netlify.com/sites/sitemap-plugin-test-site/deploys/606328dd7875ef00076150c0

[netlify-bot]

Updated Packages

• https://diff.intrinsic.com/@netlify/plugin-sitemap/0.8.0/0.8.1