Fix dead link in build.sh help message to point to correct cross-building documentation (dotnet#117325)

* Initial plan

* Fix dead link in build.sh help to point to correct cross-building documentation

Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: Copilot

eng/build.sh

@@ -117,7 +117,7 @@ usage()
   echo ""
   echo "However, for this example, you need to already have ROOTFS_DIR set up."
   echo "Further information on this can be found here:"
-  echo "https://github.com/dotnet/runtime/blob/main/docs/workflow/building/coreclr/linux-instructions.md"
+  echo "https://github.com/dotnet/runtime/blob/main/docs/workflow/building/coreclr/cross-building.md"
   echo ""
   echo "* Build Mono runtime for Linux x64 on Release configuration."
   echo "./build.sh mono -c release"