mdbook-spec is no longer needed to be installed separately.

Author: ehuss

README.md

@@ -12,7 +12,6 @@ what we have for now.
 
 - Nightly Rust
 - [mdbook](https://rust-lang.github.io/mdBook/)
-- [`mdbook-spec`](https://github.com/rust-lang/spec/tree/main/mdbook-spec)
 
 ## Installing dependencies
 
@@ -30,12 +29,6 @@ build the Reference:
 cargo install --locked mdbook
 ```
 
-Also install `mdbook-spec` which is a preprocessor which adds some Markdown extensions:
-
-```sh
-cargo install --locked mdbook-spec
-```
-
 ## Building
 
 To build the Reference, first clone the project:
@@ -78,5 +71,3 @@ You can also open any current build of the reference by running:
 ```sh
 mdbook serve --open
 ```
-
-