atrace: fix the help string

Change-Id: I19f0093d65d64e669ad71c600c103f4080bfa2c2
diff --git a/cmds/atrace/atrace.cpp b/cmds/atrace/atrace.cpp
index a4bf674..c6b3dce 100644
--- a/cmds/atrace/atrace.cpp
+++ b/cmds/atrace/atrace.cpp
@@ -562,6 +562,8 @@
                     "  --async_dump    dump the current contents of circular trace buffer\n"
                     "  --async_stop    stop tracing and dump the current contents of circular\n"
                     "                    trace buffer\n"
+                    "  --list_categories\n"
+                    "                  list the available tracing categories\n"
             );
 }