Use mdbook v0.4.21

On rust 1.64.0 mdbook v0.4.18 fails to compile:

       Compiling mdbook v0.4.18
    error[E0597]: `local_ctx` does not live long enough
       --> /Users/erichodel/.cargo/registry/src/github.com-1ecc6299db9ec823/mdbook-0.4.18/src/renderer/html_handlebars/helpers/navigation.rs:154:25
        |
    154 |             t.render(r, &local_ctx, &mut local_rc, out)
        |                         ^^^^^^^^^^ borrowed value does not live long enough
    155 |         })?;
        |         -
        |         |
        |         `local_ctx` dropped here while still borrowed
        |         borrow might be used here, when `local_rc` is dropped and runs the destructor for type `handlebars::RenderContext<'_, '_>`
        |
        = note: values in a scope are dropped in the opposite order they are defined

    For more information about this error, try `rustc --explain E0597`.
    error: could not compile `mdbook` due to previous error
    warning: build failed, waiting for other jobs to finish...

1 files changed, 1 insertions, 1 deletions

diff --git a/Cargo.toml b/Cargo.toml
index b21ffc8..119e44a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,7 +9,7 @@ repository = "https://github.com/badboy/mdbook-mermaid"
 edition = "2018"
 
 [dependencies]
-mdbook = { version = "0.4.10", default-features = false }
+mdbook = { version = "0.4.21", default-features = false }
 pulldown-cmark = { version = "0.9.0", default-features = false }
 env_logger = "0.9.0"
 log = "0.4.11"