Add missing file extension for codeowner (smithy-lang#411)

Author: gosar

.github/CODEOWNERS

@@ -9,6 +9,6 @@ smithy-typescript-integ-tests/* @adamthom-amzn @gosar @JordonPhillips
 
 # These are all specific to SSDK functionality.
 smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/ServerCommandGenerator.java @adamthom-amzn @gosar @JordonPhillips
-smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/ServerErrorGenerator @adamthom-amzn @gosar @JordonPhillips
+smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/ServerErrorGenerator.java @adamthom-amzn @gosar @JordonPhillips
 smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/ServerGenerator.java @adamthom-amzn @gosar @JordonPhillips
 smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/ServerSymbolVisitor.java @adamthom-amzn @gosar @JordonPhillips