[WeakLookupTable] Fix access modifier for subscript

We don't need to make it public.

Author: rintaro

Sources/SwiftSyntax/WeakLookupTable.swift

@@ -207,7 +207,7 @@ class WeakLookupTable<Element: Identifiable & AnyObject> {
 
   /// Get a object with specified id. Returns 'nil' if the object hasn't been
   /// insert()-ed or it's already been freed.
-  public subscript(id: Element.Identifier) -> Element? {
+  subscript(id: Element.Identifier) -> Element? {
     // Since we don't fill the bucket when the object is freed (because we don't
     // know), we can't stop iteration at a hole. So in the worst case (i.e. if
     // the object doesn't exist in the table), full linear search is needed.