fix typo

Author: RalfJung

README.md

@@ -291,7 +291,7 @@ extern "Rust" {
     /// ```
     ///
     /// The fields must be declared in exactly the same order as they appear in `MiriFrame` above.
-    /// This function can be called on any thread (not just the one which obtained `frame`)
+    /// This function can be called on any thread (not just the one which obtained `frame`).
     fn miri_resolve_frame(frame: *mut (), flags: u64) -> MiriFrame;
 
     /// Miri-provided extern function to begin unwinding with the given payload.