AbstractSingletonService: fix bug in initInstances

The last commit introduced a logic error: it missed one usage of the
"instances" variable.

Author: ctrueden

src/main/java/org/scijava/plugin/AbstractSingletonService.java

@@ -118,7 +118,7 @@ private synchronized void initInstances() {
 		final HashMap<Class<? extends PT>, PT> map =
 			new HashMap<Class<? extends PT>, PT>();
 
-		for (final PT plugin : instances) {
+		for (final PT plugin : list) {
 			@SuppressWarnings("unchecked")
 			final Class<? extends PT> ptClass =
 				(Class<? extends PT>) plugin.getClass();