readme: fix the invalid "configuration flags" link

README.md

@@ -130,15 +130,15 @@ is not automatically supported since, from the target name alone, we cannot dedu
 which JavaScript interface should be used (or if JavaScript is available at all).
 
 To enable `getrandom`'s functionality on `wasm32-unknown-unknown` using the Web
-Crypto methods [described above](#opt-in-backends) via [`wasm-bindgen`], do
+Crypto methods [described above][opt-in] via [`wasm-bindgen`], do
 *both* of the following:
 
 -   Use the `wasm_js` feature flag, i.e.
     `getrandom = { version = "0.3", features = ["wasm_js"] }`.
     On its own, this only makes the backend available. (As a side effect this
     will make your `Cargo.lock` significantly larger if you are not already
     using [`wasm-bindgen`], but otherwise enabling this feature is harmless.)
--   Set `RUSTFLAGS='--cfg getrandom_backend="wasm_js"'` ([see above](#opt-in-backends)).
+-   Set `RUSTFLAGS='--cfg getrandom_backend="wasm_js"'` ([see above][opt-in]).
 
 This backend supports both web browsers (main window and Web Workers)
 and Node.js (v19 or later) environments.
@@ -147,9 +147,10 @@ and Node.js (v19 or later) environments.
 
 If this crate does not support your target out of the box or you have to use
 a non-default entropy source, then you can provide a custom implementation.
-You need to enable the custom backend as described in the [configuration flags]
-section. Next, you need to define an `extern` function with the following
-signature:
+You need to enable the custom backend as described in the
+[opt-in backends][opt-in] section.
+
+Next, you need to define an `extern` function with the following signature:
 
 ```rust
 use getrandom::Error;
@@ -375,6 +376,7 @@ dual licensed as above, without any additional terms or conditions.
 [platform-support]: https://doc.rust-lang.org/stable/rustc/platform-support.html
 [WASI]: https://github.com/WebAssembly/WASI
 [Emscripten]: https://emscripten.org
+[opt-in]: #opt-in-backends
 
 [//]: # (licenses)