backticks are not pretty when not rendered (#2529)

These ones are particularly uncomfortable due to being of large font,
in addition to being visible on all pages of the book.

Author: tshepang

book/book.toml

@@ -1,5 +1,5 @@
 [book]
-title = "The `bindgen` User Guide"
+title = "The bindgen User Guide"
 author = "The Servo project developers"
 description = "`bindgen` automatically generates Rust FFI bindings to C and C++ libraries."