[OpenMP][Docs] Fixed a typo in the doc that can mislead users to a CMake error

When setting `LLVM_ENABLE_RUNTIMES`, lower case word should be used;
otherwise, it can cause a CMake error that specific path is not found.

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D93719

GitOrigin-RevId: 1eb082c2ea426f1dab4d1b3541b37c883b3a6b4f
1 file changed