Update obsolete comment in InputDecorator test (flutter#151651)

Update one comment which is obsolete since flutter#146754

Author: bleroux

packages/flutter/test/material/input_decorator_test.dart

@@ -4536,7 +4536,7 @@ void main() {
     const double helperGap = 4.0;
     const double helperHeight = 16.0;
     const double containerHeight = topPadding + floatingLabelHeight + labelInputGap + inputHeight + bottomPadding; // 56.0
-    const double fullHeight = containerHeight + helperGap + helperHeight; // 80.0 (should be 76.0 based on M3 spec)
+    const double fullHeight = containerHeight + helperGap + helperHeight; // 76.0
     const double errorHeight = helperHeight;
     // TODO(bleroux): consider changing this padding because, from the M3 specification, it should be 16.
     const double helperStartPadding = 12.0;