fix tinyxml2 namespaces' ending comment

[ssmolov]

Several code analysers, like clang-tidy, report warnings on ending comment for tinyxml2 namespace. Here is a clang-tidy 12.0.0 log fragment:

../tinyxml2/tinyxml2.h:2377:2: warning: namespace 'tinyxml2' ends with an unrecognized comment [llvm-namespace-comment]
}       // tinyxml2
 ^~~~~~~~~~~~~~~~~~
  // namespace tinyxml2
../tinyxml2/tinyxml2.h:114:11: note: namespace 'tinyxml2' starts here
namespace tinyxml2
          ^

This pull request is aimed to fix the issue.