Fix an obvious typo in an attribute's diagnostics.

Patch by Caitlin Sadowski.

Unfortunately, this attribute doesn't seem to have a single test. It is
only mentioned in comments in one test, and as a string literal in
a copy of some Clang code checked in as a test for the Indexer. =[ It
dates from 2009 r74280 as part of OpenCL 1.0.

llvm-svn: 134136