Add MIL.Common import to intro example

[gaearon]

I started reading MIL and was confused by the very first example not working.

I pasted this into the Lean file but got this:

Not saying this is the best possible fix pedagogically...

A few alternative suggestions:

• Could avoid using the shortcut notation until the next chapter
• Could add import MIL.Common to S01_Getting_Started.lean in the project repo instead to give a hint