Fix typo in doc comment (#154)

diff --git a/lib/src/arena.dart b/lib/src/arena.dart
index cc6fd0b..747d120 100644
--- a/lib/src/arena.dart
+++ b/lib/src/arena.dart
@@ -136,7 +136,7 @@
 
 /// Creates a zoned [Arena] to manage native resources.
 ///
-/// The arena is availabe through [zoneArena].
+/// The arena is available through [zoneArena].
 ///
 /// If the isolate is shut down, through `Isolate.kill()`, resources are _not_
 /// cleaned up.