Updated index.md. Added a command which can show an unoptimized MIR

The current playground link used in the page of MIR shows a optimized version of MIR which is missing some statements such as StorageLive . Updated to use a local command which shows unoptimized MIR that would be more useful for pedagogical purposes.

Author: Prashant-Shekhar-Rao

src/mir/index.md

@@ -62,7 +62,7 @@ show you the MIR for your program. Try putting this program into play
 button on the top:
 
 [sample-play]: https://play.rust-lang.org/?gist=30074856e62e74e91f06abd19bd72ece&version=stable
-
+MIR shown by link is missing some statements for unoptimized MIR use `-Z mir-opt-level=0 --emit mir`
 ```rust
 fn main() {
     let mut vec = Vec::new();