[libcxx] [test] Fix comment typos.

No functional change, no code review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294160 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp

@@ -96,7 +96,7 @@ int main()
     }
     {
         // Test that the uses-allocator default constructor does not evaluate
-        // it's SFINAE when it otherwise shouldn't be selected. Do this by
+        // its SFINAE when it otherwise shouldn't be selected. Do this by
         // using 'NonDefaultConstructible' which will cause a compile error
         // if std::is_default_constructible is evaluated on it.
         using T = NonDefaultConstructible<>;