[chai] Correct type of AssertionError

AssertionError on the global chai object is the constructor for AssertionErrors, but the definition was written as though it was an instance of an assertion error.

chai/chai.d.ts

@@ -19,7 +19,7 @@ declare module Chai {
         use(fn: (chai: any, utils: any) => void): any;
         assert: AssertStatic;
         config: Config;
-        AssertionError: AssertionError;
+        AssertionError: typeof AssertionError;
     }
 
     export interface ExpectStatic extends AssertionStatic {