Update doctest_impl.h (#112)

Fix macros `WIN32_LEAN_AND_MEAN` typo
diff --git a/doctest/parts/doctest_impl.h b/doctest/parts/doctest_impl.h
index 17ae9d6..9295f7a 100644
--- a/doctest/parts/doctest_impl.h
+++ b/doctest/parts/doctest_impl.h
@@ -554,9 +554,9 @@
 #ifdef DOCTEST_PLATFORM_WINDOWS
 
 // defines for a leaner windows.h
-#ifndef WIN32_MEAN_AND_LEAN
-#define WIN32_MEAN_AND_LEAN
-#endif // WIN32_MEAN_AND_LEAN
+#ifndef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#endif // WIN32_LEAN_AND_MEAN
 #ifndef VC_EXTRA_LEAN
 #define VC_EXTRA_LEAN
 #endif // VC_EXTRA_LEAN