Fix --compiler-backend usage example

Change-Id: I2ebefe57e94d6a8103c71e86f7c6a302b7107fc4
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 1beb862..98c62ce 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -133,7 +133,7 @@
   UsageError("");
   UsageError("  --compiler-backend=(Quick|QuickGBC|Portable): select compiler backend");
   UsageError("      set.");
-  UsageError("      Example: --instruction-set=Portable");
+  UsageError("      Example: --compiler-backend=Portable");
   UsageError("      Default: Quick");
   UsageError("");
   UsageError("  --host: used with Portable backend to link against host runtime libraries");