fix REPLACE syntax, the collection was missing. (#4705)

Documentation/Books/AQL/Operations/Replace.md

@@ -125,6 +125,7 @@ returned:
 ```
 FOR u IN users
   REPLACE u WITH { value: "test" } 
+  IN users
   LET previous = OLD 
   RETURN previous._key
 ```
@@ -134,7 +135,7 @@ documents (without some of their system attributes):
 
 ```
 FOR u IN users
-  REPLACE u WITH { value: "test" } 
+  REPLACE u WITH { value: "test" } IN users
   LET replaced = NEW 
   RETURN UNSET(replaced, '_key', '_id', '_rev')
 ```