Update ImageJ -> ImageJ2

Author: ctrueden

README.md

@@ -5,5 +5,5 @@
 SciJava Common is a common library for SciJava software. It provides a
 plugin framework, with an extensible mechanism for service discovery, backed
 by its own annotation processor, so that plugins can be loaded dynamically.
-It is used by both [ImageJ](https://github.com/imagej/imagej) and
+It is used by both [ImageJ2](https://github.com/imagej/imagej2) and
 [SCIFIO](https://github.com/scifio/scifio).