Update description of CARGO_PKG_README

Author: chansuke

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

@@ -231,7 +231,7 @@ corresponding environment variable is set to the empty string, `""`.
 * `CARGO_PKG_RUST_VERSION` — The Rust version from the manifest of your package.
   Note that this is the minimum Rust version supported by the package, not the
   current Rust version.
-* `CARGO_PKG_README` — The location of README of your package.
+* `CARGO_PKG_README` — Path to the README file of your package.
 * `CARGO_CRATE_NAME` — The name of the crate that is currently being compiled. It is the name of the [Cargo target] with `-` converted to `_`, such as the name of the library, binary, example, integration test, or benchmark.
 * `CARGO_BIN_NAME` — The name of the binary that is currently being compiled (if it is a binary). This name does not include any file extension, such as `.exe`.
 * `OUT_DIR` — If the package has a build script, this is set to the folder where the build