Update index.md (#213)

Clarify that the value of SCRATCH discussed here is for the ML platform
to avoid confusion if someone finds this through another link and
doesn't notice it is only for the MLp.

Author: jpcoles-cscs

docs/platforms/mlp/index.md

@@ -51,7 +51,7 @@ Scratch filesystems provide temporary storage for high-performance I/O for execu
 Use scratch to store datasets that will be accessed by jobs, and for job output.
 Scratch is per user - each user gets separate scratch path and quota.
 
-* The environment variable `SCRATCH=/iopsstor/scratch/cscs/$USER` is set automatically when you log into the system, and can be used as a shortcut to access scratch.
+* The environment variable `SCRATCH=/iopsstor/scratch/cscs/$USER` is set automatically when you log into a system of the ML platform, and can be used as a shortcut to access scratch.
 * There is an additional scratch path mounted on [Capstor][ref-alps-capstor] at `/capstor/scratch/cscs/$USER`.
 
 !!! warning "scratch cleanup policy"