ICU4J: Fix grammar rule in comment

Author: catamorphism

Diff for: icu4j/main/core/src/main/java/com/ibm/icu/message2/MFParser.java

@@ -598,7 +598,7 @@ private MFDataModel.Message getComplexMessage() throws MFParseException {
         } else { // Expect {{...}} or end of message
             skipOptionalWhitespaces();
             int cp = input.peekChar();
-            // complex-message   = *(declaration [s]) complex-body
+            // complex-message   = *(declaration [s]) complex-body [s]
             checkCondition(cp != EOF, "Expected a quoted pattern or .match; got end-of-input");
             MFDataModel.Pattern pattern = getQuotedPattern();
             skipOptionalWhitespaces(); // Trailing whitespace is allowed