Corrected MIR file path and fixed a typo

[saki-osive]

In this PR I have corrected the hyperlink for MIR in the page https://github.com/rust-lang/rustc-dev-guide/blob/master/src/part-5-intro.md

So far, we've shown how the compiler can take raw source code in text format and transform it into MIR. We have also shown how the compiler does various analyses on the code to detect things like type or lifetime errors. Now, we will finally take the MIR and produce some executable machine code.

The link that originally pointed to the highlighted MIR was incorrectly set to https://github.com/rust-lang/rustc-dev-guide/blob/master/src/mir/index.html. I have set it to https://github.com/rust-lang/rustc-dev-guide/blob/master/src/mir/index.md.

Plus, I have corrected a typo in the readme.md.