[Clang] Fix `-Wdocumentation` warning (NFC)

```
llvm-project/clang/include/clang/AST/OpenMPClause.h:7762:14: warning: parameter 'Modifier' not found in the function declaration [-Wdocumentation]
  /// \param Modifier The modifier applied to 'order' clause.
             ^~~~~~~~
llvm-project/clang/include/clang/AST/OpenMPClause.h:7762:14: note: did you mean 'M'?
  /// \param Modifier The modifier applied to 'order' clause.
```