Update installation instructions for FairScale (Lightning-AI#5099)

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

docs/source/multi_gpu.rst

@@ -663,7 +663,7 @@ It is highly recommended to use Sharded Training in multi-GPU environments where
 A technical note: as batch size scales, storing activations for the backwards pass becomes the bottleneck in training. As a result, sharding optimizer state and gradients becomes less impactful.
 Work within the future will bring optional sharding to activations and model parameters to reduce memory further, but come with a speed cost.
 
-To use Sharded Training, you need to first install FairScale using the command below or install all extras using ``pip install pytorch-lightning["extra"]``.
+To use Sharded Training, you need to first install FairScale using the command below.
 
 .. code-block:: bash