message: Fixed typo in issue template
diff: @@ -16,5 +16,5 @@ Please provide the following information:
  - Build tools and configuration you used to build the engine (e.g. Visual Studio 2017 Debug x64 + CMake 3.15.4)
  - GPU and driver version you have installed on your system
  - Debug output, especially messages from Diligent
  -- Call stack (if avaialable)
  +- Call stack (if available)
  - If you experience a problem with one back-end, please try running other back-ends and see if the same issue arises
  