Platform: fix EmscriptenApplication context creation on WebGL 2.

Insufficient testing, sorry. This got broken after MAGNUM_TARGET_GLES3
deprecation in 87c7eea1e2, but the stupid
variable name was there before already, and it just worked for some
reason, but when the condition got flipped, setting *minor* version to 2
didn't do the right thing anymore. And of course, typical Emscripten,
it didn't complain at all in either case.

Took me an embarrassingly long time to spot what was wrong.

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -376,7 +376,7 @@ bool EmscriptenApplication::tryCreate(const Configuration& configuration, const
     #ifdef MAGNUM_TARGET_GLES2 /* WebGL 1 */
     attrs.majorVersion = 1;
     #else                      /* WebGL 2 */
-    attrs.minorVersion = 2;
+    attrs.majorVersion = 2;
     #endif
 
     std::ostream* verbose = _verboseLog ? Debug::output() : nullptr;