RUST-1560 Fix README typo (#798)

Author: lonesometraveler

manual/src/README.md

@@ -33,5 +33,5 @@ tokio::time::timeout(Duration::from_secs(5), handle).await???;
 
 ## Minimum supported Rust version (MSRV)
 
-The MSRV for this crate is currently 1.57.0. This will be rarely be increased, and if it ever is,
-it will only happen in a minor or major version release.
+The MSRV for this crate is currently 1.57.0. This will rarely be increased, and if it ever is,
+it will only happen in a minor or major version release.