Changed Flow syntax that was causing invalid build syntax

Author: Brian Vaughn

packages/react-reconciler/src/ReactFiberHooks.js

@@ -1784,7 +1784,7 @@ if (__DEV__) {
     ): Snapshot {
       currentHookNameInDev = 'useMutableSource';
       mountHookTypesDev();
-      return mountMutableSource<Source, Snapshot>(source, config);
+      return mountMutableSource(source, config);
     },
   };
 
@@ -2299,7 +2299,7 @@ if (__DEV__) {
       currentHookNameInDev = 'useMutableSource';
       warnInvalidHookAccess();
       mountHookTypesDev();
-      return mountMutableSource<Source, Snapshot>(source, config);
+      return mountMutableSource(source, config);
     },
   };