docs(ref): Use better example value in CARGO_CFG_TARGET_ABI (#15404)

`target_abi = "sim"` may be deprecated in the future. See
rust-lang/rust#139451.

Author: weihanglo

src/doc/src/reference/environment-variables.md

@@ -350,7 +350,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
     * `CARGO_CFG_TARGET_ARCH=x86_64` --- The CPU [target architecture].
     * `CARGO_CFG_TARGET_VENDOR=apple` --- The [target vendor].
     * `CARGO_CFG_TARGET_ENV=gnu` --- The [target environment] ABI.
-    * `CARGO_CFG_TARGET_ABI=sim` --- The [target ABI].
+    * `CARGO_CFG_TARGET_ABI=eabihf` --- The [target ABI].
     * `CARGO_CFG_TARGET_POINTER_WIDTH=64` --- The CPU [pointer width].
     * `CARGO_CFG_TARGET_ENDIAN=little` --- The CPU [target endianness].
     * `CARGO_CFG_TARGET_FEATURE=mmx,sse` --- List of CPU [target features] enabled.