Update dependency mdbook to v0.5.2

[renovate]

This PR contains the following updates:

Package	Update	Change
mdbook	patch	0.5.1 → 0.5.2

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Release Notes

rust-lang/mdBook (mdbook)

v0.5.2

Compare Source

v0.5.1...v0.5.2

Changed

• Updated Rust crate html5ever to 0.36.0.
  #​2970
• Updated cargo dependencies.
  #​2969

Fixed

• Fixed repeated error message when HTML config is invalid in mdbook serve.
  #​2983
• Fixed sidebar scroll position when heading nav is involved.
  #​2982
• Fixed color for rustdoc error messages.
  #​2981
• Fixed usage of custom preprocessors with MDBook::test.
  #​2980

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