Highlight that the code snippet will not work (#204)

Highlight that the code snippet will not work. Users most of the time will
skim read the docs trying to spot snippets on how to use the code.

Since this is the only snippet on the welcome page, we might want to
highlight that it's an example that won't work.

docs/index.rst

@@ -50,6 +50,8 @@ The core values of Injector are:
   a different configuration and each with different objects in different scopes. Code like this
   won't work for this very reason::
 
+    # This will NOT work:
+
     class MyClass:
         @inject
         def __init__(self, t: SomeType):