Use relative link for core::sync::atomic

Author: ehuss

src/conditional-compilation.md

@@ -202,7 +202,7 @@ unstable), which additionally require that the primitive integer and atomic have
 the same minimum alignment on the given target. No user-visible, stable cfg is
 exposed for that method at this time.
 
-[`core::sync::atomic`]: https://doc.rust-lang.org/nightly/core/sync/atomic/index.html
+[`core::sync::atomic`]: ../core/sync/atomic/index.html
 
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