Fix typo in method name

This method name had a typo, since the method in the full specification
is called `default_gem?` not `default_gem`. That typo being unnoticed
most likely indicates that the method was not being run at all. However,
since I intend to use it in later commits, I'm fixing the typo and not
removing it.

bundler/lib/bundler/stub_specification.rb

@@ -41,8 +41,8 @@ def activated=(activated)
       stub.instance_variable_set(:@activated, activated)
     end
 
-    def default_gem
-      stub.default_gem
+    def default_gem?
+      stub.default_gem?
     end
 
     def full_gem_path