Fix a broken documentation link

[alexander-jackson]

Fix a broken documentation link that should point to serde_qs so that
users do not have to search themselves.

Description

Simply fix a broken documentation link referring to the external serde_qs crate so that users can click it on docs.rs instead of searching for the crate themselves.

Motivation and Context

This allows the documentation to be followed slightly more easily and means cargo doc produces no warnings (other than those created by this issue).

How Has This Been Tested?

This affects no tests and makes no changes to the functionality, so the unit tests pass before and after the change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.