nixos/ollama: fix `rocmOverrideGfx` description (#337696)

        example = "10.3.0";

        description = ''

          Override what rocm will detect your gpu model as.

          For example, make rocm treat your RX 5700 XT (or any other model)

          as an RX 6900 XT using a value of `"10.3.0"` (gfx 1030).

          For example, if you have an RX 5700 XT, try setting this to `"10.1.0"` (gfx 1010).

          This sets the value of `HSA_OVERRIDE_GFX_VERSION`. See [ollama's docs](

          https://github.com/ollama/ollama/blob/main/docs/gpu.md#amd-radeon