Auto merge of rust-lang#10216 - chansuke:hotfix/fix-spelling, r=llogiq

Fix spelling inconsistence of `mdBook`

I was reading both [mdBook Documentation](https://rust-lang.github.io/mdBook/) and  [The Clippy Book](https://doc.rust-lang.org/nightly/clippy/development/infrastructure/book.html#the-clippy-book) and felt that it's easier to read to unify the notation of `mdBook`.

- \[ ] Followed [lint naming conventions][lint_naming]
- \[ ] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[ ] Executed `cargo dev update_lints`
- \[ ] Added lint documentation
- \[x] Run `cargo dev fmt`

----

changelog: none

Author: bors

book/src/development/infrastructure/book.md

@@ -3,31 +3,31 @@
 This document explains how to make additions and changes to the Clippy book, the
 guide to Clippy that you're reading right now. The Clippy book is formatted with
 [Markdown](https://www.markdownguide.org) and generated by
-[mdbook](https://github.com/rust-lang/mdBook).
+[mdBook](https://github.com/rust-lang/mdBook).
 
-- [Get mdbook](#get-mdbook)
+- [Get mdBook](#get-mdbook)
 - [Make changes](#make-changes)
 
-## Get mdbook
+## Get mdBook
 
 While not strictly necessary since the book source is simply Markdown text
-files, having mdbook locally will allow you to build, test and serve the book
+files, having mdBook locally will allow you to build, test and serve the book
 locally to view changes before you commit them to the repository. You likely
 already have `cargo` installed, so the easiest option is to simply:
 
 ```shell
 cargo install mdbook
 ```
 
-See the mdbook [installation](https://github.com/rust-lang/mdBook#installation)
+See the mdBook [installation](https://github.com/rust-lang/mdBook#installation)
 instructions for other options.
 
 ## Make changes
 
 The book's
 [src](https://github.com/rust-lang/rust-clippy/tree/master/book/src)
 directory contains all of the markdown files used to generate the book. If you
-want to see your changes in real time, you can use the mdbook `serve` command to
+want to see your changes in real time, you can use the mdBook `serve` command to
 run a web server locally that will automatically update changes as they are
 made. From the top level of your `rust-clippy` directory:
 
@@ -38,5 +38,5 @@ mdbook serve book --open
 Then navigate to `http://localhost:3000` to see the generated book. While the
 server is running, changes you make will automatically be updated.
 
-For more information, see the mdbook
+For more information, see the mdBook
 [guide](https://rust-lang.github.io/mdBook/).