doc: use correct minimum Rust version

Detected via 'cargo msrv find' using the cargo-msrv crate.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Author: tycho

README.md

@@ -277,4 +277,4 @@ cargo build --release  # release
 cargo test
 ```
 
-Requires Rust 1.87 or later.
+Requires Rust 1.88 or later.