[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

llvm-svn: 290766