fix: rename incorrectly documented target-spec-json to json-target-sp… (rust-lang#16905)

Close rust-lang#16896.

Author: weihanglo

src/doc/src/reference/unstable.md

@@ -100,7 +100,7 @@ Each new feature described below should explain how to use it.
     * [panic-immediate-abort](#panic-immediate-abort) --- Passes `-Cpanic=immediate-abort` to the compiler.
     * [compile-time-deps](#compile-time-deps) --- Perma-unstable feature for rust-analyzer
     * [fine-grain-locking](#fine-grain-locking) --- Use fine grain locking instead of locking the entire build cache
-    * [target-spec-json](#target-spec-json) --- Allows the use of `.json` custom target specs.
+    * [json-target-spec](#json-target-spec) --- Allows the use of `.json` custom target specs.
 * rustdoc
     * [rustdoc-map](#rustdoc-map) --- Provides mappings for documentation to link to external sites like [docs.rs](https://docs.rs/).
     * [scrape-examples](#scrape-examples) --- Shows examples within documentation.
@@ -2018,13 +2018,13 @@ so that `cargo doc` can merge cross-crate information
 from separate output directories,
 and run `rustdoc` in parallel.
 
-## target-spec-json
+## json-target-spec
 * Tracking Issue: [rust-lang/rust#151528](https://github.com/rust-lang/rust/issues/151528)
 
-The `-Z target-spec-json` CLI flag enables the ability to use [custom target spec JSON files](https://doc.rust-lang.org/nightly/rustc/targets/custom.html) as a target.
+The `-Z json-target-spec` CLI flag enables the ability to use [custom target spec JSON files](https://doc.rust-lang.org/nightly/rustc/targets/custom.html) as a target.
 
 ```console
-cargo +nightly build --target my-target.json -Z target-spec-json
+cargo +nightly build --target my-target.json -Z json-target-spec
 ```
 
 This usually must be combined with [build-std](#build-std).