pkg/isolate: spelling fixes

README.md

 # isolate
 
 The `isolate` package helps with isolates and isolate communication.
 
 The package contains individual libraries with different purposes.
 
 ### Creating send ports and responding to messages.
 
 The "ports.dart" sub-library contains functionality
 for creating `SendPort`s and reacting to values sent to those ports.
 
 ### Working with isolates and running functions in other isolates.
 
 The "isolate_runner.dart" sub-library introduces an `IsolateRunner` class
 that gives easy access to the `Isolate` functionality, and also
 gives a way to run new functions in the isolate repeatedly, instead of
 just on the initial `spawn` call.
 
-### A central registry for values that can be used accross isolates.
+### A central registry for values that can be used across isolates.
 
 The "registry.dart" sub-library provides a way to create an
-object registry, and give access to it accross different isolates.
+object registry, and give access to it across different isolates.
 
-### Balancing load accross several isolates.
+### Balancing load across several isolates.
 
 The "load_balancer.dart" sub-library can manage multiple `Runner` objects,
 including `IsolateRunner`, and run functions on the currently least loaded
 runner.
 
 ## Features and bugs
 
 Please file feature requests and bugs at the [issue tracker][tracker].
 
 [tracker]: https://github.com/dart-lang/isolate/issues