docs: fix typo in tracing/README.md (#1305)

tracing/README.md

@@ -108,7 +108,7 @@ In addition, you can locally override the default subscriber. For example:
 
 ```rust
 use tracing::{info, Level};
-use tracing_subscruber::FmtSubscriber;
+use tracing_subscriber::FmtSubscriber;
 
 fn main() {
     let subscriber = tracing_subscriber::FmtSubscriber::builder()