[clang] Fix flag typo in comment

Fixed for more accurate searches of the flag `-Wsystem-headers-in-module=`.

Author: kastiglione

clang/include/clang/Basic/DiagnosticOptions.h

@@ -124,7 +124,7 @@ class DiagnosticOptions : public RefCountedBase<DiagnosticOptions>{
   /// default).
   std::vector<std::string> VerifyPrefixes;
 
-  /// The list of -Wsystem-header-in-module=... options used to override
+  /// The list of -Wsystem-headers-in-module=... options used to override
   /// whether -Wsystem-headers is enabled on a per-module basis.
   std::vector<std::string> SystemHeaderWarningsModules;