fix typo in ASTWalker

Author: deafgoat

lib/AST/ASTWalker.cpp

@@ -21,8 +21,8 @@
 //  a node:
 //
 //    - Every node should be walked.  If a node has both syntactic and
-//      semantic components, you should make you visit every node in
-//      both.
+//      semantic components, you should make sure you visit every node
+//      in both.
 //
 //    - Nodes should only be walked once.  So if a node has both
 //      syntactic and semantic components, but the type-checker builds