Fix doxygen comment

Change-Id: I5977c7fe09f80e9da9438e7842fdfdf44346c839
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/37220
Commit-Queue: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: David Neto <dneto@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/writer/wgsl/generator_impl.h

@@ -53,7 +53,7 @@ namespace wgsl {
 class GeneratorImpl : public TextGenerator {
  public:
   /// Constructor
-  /// @param module the module to generate
+  /// @param mod the module to generate
   explicit GeneratorImpl(ast::Module* mod);
   ~GeneratorImpl();