Fix a comment for ValueObject::GetValueDidChange after r231526

Summary:
Fix a comment for ValueObject::GetValueDidChange after r231526.

This fix was requested by @jingham.

Reviewers: jingham, ki.stfu

Reviewed By: ki.stfu

Subscribers: lldb-commits, jingham

Differential Revision: http://reviews.llvm.org/D8206

llvm-svn: 231804