Clarify maybe_unused test comment

Put the C++17 extractor option on the first line and name the maybe_unused attribute in the test expectation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -1,6 +1,5 @@
 
 // semmle-extractor-options: -std=c++17
-
 int globalVar; // GOOD (not static)
 static int staticVar1; // GOOD (used)
 static int staticVar2; // GOOD (used)
@@ -12,7 +11,7 @@ static __attribute__((__unused__)) int staticVar7; // GOOD (unused but this is e
 const int constVar8 = 8; // $ Alert // BAD (const defaults to static)
 extern const int constVar9 = 9; // GOOD
 static int staticVar10 = 10; // $ SPURIOUS: Alert // GOOD [FALSE POSITIVE] (referenced in a never instantiated template)
-[[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as unused)
+[[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as maybe_unused)
 
 void f()
 {