Fix error code

Author: Lamparter

src/roslyn/Riverside.CompilerPlatform.SourceGenerators/IncrementalGenerator.cs

@@ -251,7 +251,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
 						// Report exceptions that occur during generation
 						sourceProductionContext.ReportDiagnostic(
 							CreateDiagnostic(
-								$"RX9999",
+								$"RS9999",
 								"Source Generation Error",
 								$"An error occurred during source generation: {ex.Message}",
 								DiagnosticSeverity.Error));