Fix typo in downcast template specialization in WeakPtr

https://bugs.webkit.org/show_bug.cgi?id=255578

Reviewed by Chris Dumez.

Fix the typo in the specialization I introduced. Verified that using the
new downcast on a WeakPtr builds fine.

* Source/WTF/wtf/WeakPtr.h:
(WTF::downcast):

Canonical link: https://commits.webkit.org/263058@main

Source/WTF/wtf/WeakPtr.h

@@ -341,7 +341,7 @@ inline bool is(const WeakPtr<ArgType, WeakPtrImpl>& source)
 template<typename Target, typename Source, typename WeakPtrImpl>
 inline Target* downcast(WeakPtr<Source, WeakPtrImpl>& source)
 {
-    return downcase<Target>(source.get());
+    return downcast<Target>(source.get());
 }
 
 template<typename Target, typename Source, typename WeakPtrImpl>