[AST][NFC] Silence GCC warning about multiline comments

Remove continuation line in code snippet to prevent GCC warning about
multiline comments (-Wcomment) when building a project using libclang
with GCC.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D92409

clang/include/clang/AST/DeclOpenMP.h

@@ -163,7 +163,7 @@ public:
 /// 'float':
 ///
 /// \code
-/// #pragma omp declare reduction (foo : int,float : omp_out += omp_in) \
+/// #pragma omp declare reduction (foo : int,float : omp_out += omp_in)
 ///                     initializer (omp_priv = 0)
 /// \endcode
 ///