Correctly document cranelift_opt_level's default

The default has been "speed" for quite some time now, I'm not sure I
remember a time when it was `None`...

Author: alexcrichton

crates/wasmtime/src/config.rs

@@ -1192,7 +1192,7 @@ impl Config {
     /// optimization level used for generated code in a few various ways. For
     /// more information see the documentation of [`OptLevel`].
     ///
-    /// The default value for this is `OptLevel::None`.
+    /// The default value for this is `OptLevel::Speed`.
     #[cfg(any(feature = "cranelift", feature = "winch"))]
     pub fn cranelift_opt_level(&mut self, level: OptLevel) -> &mut Self {
         let val = match level {