typo

doc/extending/graphstructures.txt

@@ -386,9 +386,9 @@ The graph gets cloned at the start of compilation, so modifications done
 during compilation won't affect the user graph.
 
 Each variable receives a new field called clients. It is a list with
-reference to every place in the graph where this variable is used. If
+references to every place in the graph where this variable is used. If
 its length is 0, it means the variable isn't used. Each place where it
-is used is described by a tuple of 2 elements. There is two types of
+is used is described by a tuple of 2 elements. There are two types of
 pairs:
 
 - The first element is an Apply node.