Editorial: Change ref. to Object.assign: method => function. (#464)

Being consistent with earlier in the clause.

spec.html

@@ -22600,7 +22600,7 @@ <h1>Object.assign ( _target_, ..._sources_ )</h1>
                 1. Perform ? Set(_to_, _nextKey_, _propValue_, *true*).
           1. Return _to_.
         </emu-alg>
-        <p>The `length` property of the `assign` method is 2.</p>
+        <p>The `length` property of the `assign` function is 2.</p>
       </emu-clause>
 
       <!-- es6num="19.1.2.2" -->