[libc++][docs][NFC] Update supported OpenXL version (#127072)

According to the documentation:
The base LLVM version was changed between minor versions: 
- 17.1.2 is based on LLVM17 
- 17.1.3 is based on LLVM19 
https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.2?topic=whats-new
https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.3?topic=whats-new

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/docs/index.rst

@@ -133,7 +133,7 @@ Compiler     Versions            Restrictions               Support policy
 ============ =================== ========================== =====================
 Clang        18, 19, 20, 21-git                             latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
 AppleClang   15                                             latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
-Open XL      17.1 (AIX)                                     latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
+Open XL      17.1.3 (AIX)                                   latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
 GCC          14                  In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
 ============ =================== ========================== =====================