docs: update MSRV in readme

Author: robjtede

README.md

@@ -9,7 +9,7 @@
 <br />
 [![CI](https://github.com/x52dev/double-int/actions/workflows/ci.yml/badge.svg)](https://github.com/x52dev/double-int/actions/workflows/ci.yml)
 [![codecov](https://codecov.io/gh/x52dev/double-int/branch/main/graph/badge.svg)](https://codecov.io/gh/x52dev/double-int)
-![Version](https://img.shields.io/badge/rustc-1.65+-ab6000.svg)
+![Version](https://img.shields.io/badge/rustc-1.56+-ab6000.svg)
 [![Download](https://img.shields.io/crates/d/double-int.svg)](https://crates.io/crates/double-int)
 
 <!-- prettier-ignore-end -->