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

Cargo.toml

@@ -5,6 +5,7 @@ edition        = "2024"
 description    = "Loader generator for Vulkan, OpenGL, OpenGL ES, EGL, GLX, and WGL"
 license        = "MIT OR Apache-2.0"
 repository     = "https://github.com/tycho/gloam"
+rust-version   = "1.88"
 
 [[bin]]
 name           = "gloam"

README.md

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