Add missing tracking issue number for --print=check-cfg

Author: Urgau

src/doc/unstable-book/src/compiler-flags/print-check-cfg.md

@@ -1,6 +1,6 @@
 # `print=check-cfg`
 
-The tracking issue for this feature is: [#XXXXXX](https://github.com/rust-lang/rust/issues/XXXXXX).
+The tracking issue for this feature is: [#125704](https://github.com/rust-lang/rust/issues/125704).
 
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