fix: correct Fiber instantiation syntax in README code example

The example previously used `new Fiber { ... }` which is incorrect Wren syntax; changed to `Fiber.new { ... }` to match the language's constructor pattern.

README.md

@@ -12,7 +12,7 @@ class Wren {
   }
 }
 
-var adjectives = new Fiber {
+var adjectives = Fiber.new {
   ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
 }