Fix link to the prepared Docker images with Rust for Lambda

[zamazan4ik]

Hi!

I want to propose a slight change to the README file. You reference the https://github.com/softprops/lambda-rust repo as a repo with lambda-rust Docker images. Unfortunately, this repo is in an unmaintained-like state (see softprops/lambda-rust#99).

We (me and other people) created a new org, where we've continued preparing related Docker images for newer Rust versions - https://github.com/rust-serverless/lambda-rust

So I suggest at least add a note about the new place where people can grab Docker images for newer Rust versions.

Also, will be awesome, if such a useful thing like Docker images, will be a part for awslabs organization, so they always will be in a maintained state (I hope). And a chance of yet another transition will be reduced. What do you think?

Thanks!

[ytitov]

Thank you for the updated repo. It finally worked for me. @zamazan4ik