Correct assert text in DeclGroup::getSingleDecl()

Assert text for getSingleDecl() is inaccurate. Appears to have been copy pasted
from getDeclGroup().

Patch by Ben Taylor!

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24518


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281525 91177308-0d34-0410-b5e6-96231b3b80d8