Fix ASTERIXDB-1043

The example for the index-drop statement was wrong. It only used a
Qualified name instead of a DoubleQualified

Change-Id: I762eadd60a7b9e09be244d8eaeb673dd03eb2108
Reviewed-on: https://asterix-gerrit.ics.uci.edu/514
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Till Westmann <tillw@apache.org>
diff --git a/asterix-doc/src/site/markdown/aql/manual.md b/asterix-doc/src/site/markdown/aql/manual.md
index af45f76..883e285 100644
--- a/asterix-doc/src/site/markdown/aql/manual.md
+++ b/asterix-doc/src/site/markdown/aql/manual.md
@@ -793,7 +793,7 @@
 
     drop dataset FacebookUsers if exists;
 
-    drop index fbSenderLocIndex;
+    drop index FacebookUsers.fbSenderLocIndex;
 
     drop type FacebookUserType;