[Docs] Update installation instructions to include CUDA 11.8 xFormers (#2246)

Author: skt7

docs/source/getting_started/installation.rst

@@ -42,6 +42,10 @@ You can install vLLM using pip:
         $ pip uninstall torch -y
         $ pip install torch --upgrade --index-url https://download.pytorch.org/whl/cu118
 
+        $ # Re-install xFormers with CUDA 11.8.
+        $ pip uninstall xformers -y
+        $ pip install --upgrade xformers --index-url https://download.pytorch.org/whl/cu118
+
 
 .. _build_from_source: