docs: Fix the confusing example of server.proxy

[rxliuli]

If you write the actual requested url like this, you will spell /foo again. For example, the actual mapped url of /foo/get is http://localhost:4567/foo/foo/get, which will lead to 404 unless the user Reuse rewrite configuration

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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