typo fix

Author: changkhothuychung

clang/include/clang/Parse/Parser.h

@@ -5164,7 +5164,7 @@ class Parser : public CodeCompletionHandler {
   /// ParseCXXReflectExpression - parses the operand of reflection operator
   /// \param DoubleCaretLoc the location of the double caret operator
   ///
-  /// \returns on success, a expression holding the constructed CXXReflectExpr;
+  /// \returns on success, an expression holding the constructed CXXReflectExpr;
   ///          on failture, an ExprError
   ExprResult ParseCXXReflectExpression(SourceLocation DoubleCaretLoc);