Update lib/src/builder.dart

Co-authored-by: Nate Bosch <nbosch1@gmail.com>
diff --git a/lib/src/builder.dart b/lib/src/builder.dart
index a9d53f7..bacc01a 100644
--- a/lib/src/builder.dart
+++ b/lib/src/builder.dart
@@ -2337,7 +2337,7 @@
           'should be a string ending with `.dart`');
     }
     if (output is! String || !output.endsWith('.mocks.dart')) {
-      throw ArgumentError('Invalid key in build_extensions `$output`, it '
+      throw ArgumentError('Invalid value in build_extensions `$output`, it '
           'should be a string ending with `mocks.dart`');
     }
     result[input] = [output];