[clang] Minor fix to libASTMatcherTutorial

There was a small error in the code in the tutorial. The tutorial contains a few errors which results in code not being able to compile.

One error was described here: https://llvm.org/bugs/show_bug.cgi?id=25583 .

I found and fixed the error and one additional error.

Reviewers: aaron.ballman, malcolm.parsons

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28180

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290766 91177308-0d34-0410-b5e6-96231b3b80d8