Add `ONOS` to dictionary and fix usage in text blocks

Change-Id: I9eabdacfcff34c8e730c0148e1e6ce078cff796a
diff --git a/troubleshooting.rst b/troubleshooting.rst
index 6c0d74e..fdc0764 100644
--- a/troubleshooting.rst
+++ b/troubleshooting.rst
@@ -59,7 +59,7 @@
 ..
     TODO Hung-Wei
 
-onos-diagnostics
+ONOS diagnostics
 ----------------
 ..
     TODO Hung-Wei