Update README.rst to reflect actual MSRV 1.74 (#1119)

The CHANGELOG indicates that MSRV was increased to 1.74. The README was never updated.

Author: cpburnz

README.rst

@@ -22,7 +22,7 @@ To install bcrypt, simply:
     $ pip install bcrypt
 
 Note that bcrypt should build very easily on Linux provided you have a C
-compiler and a Rust compiler (the minimum supported Rust version is 1.56.0).
+compiler and a Rust compiler (the minimum supported Rust version is 1.74.0).
 
 For Debian and Ubuntu, the following command will ensure that the required dependencies are installed: