[libc++][NFC] Fixed bad link in 21.rst (#130428)

Author: H-G-Hristov

libcxx/docs/ReleaseNotes/21.rst

@@ -41,7 +41,7 @@ Implemented Papers
 - N4258: Cleaning-up noexcept in the Library (`Github <https://github.com/llvm/llvm-project/issues/99937>`__)
 - P0767R1: Deprecate POD (`Github <https://github.com/llvm/llvm-project/issues/104013>`__)
 - P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
-- P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`)
+- P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`__)
 
 Improvements and New Features
 -----------------------------