Fix [env] relative description in reference

Author: MingweiSamuel

src/doc/src/reference/config.md

@@ -89,7 +89,7 @@ browser = "chromium"          # browser to use with `cargo doc --open`,
 ENV_VAR_NAME = "value"
 # Set even if already present in environment
 ENV_VAR_NAME_2 = { value = "value", force = true }
-# Value is relative to .cargo directory containing `config.toml`, make absolute
+# Value is relative to the parent where `.cargo/config.toml` is located
 ENV_VAR_NAME_3 = { value = "relative/path", relative = true }
 
 [future-incompat-report]