Update installation.rst

Author: CPernet

docs/installation.rst

@@ -46,7 +46,7 @@ In short, install the ``petprep-docker`` wrapper with pip::
 
   $ python -m pip install petprep-docker
 
-Warning - since petprep relies on many tools (see below Dependencies), you need at least 50 Gb free space (also beware of docker settings which by default will not use the last 10% of your disk space).
+Warning - since petprep relies on many tools (see below Dependencies), you need about 30 Gb free space (the image is 24GB -- also beware of docker settings which by default will not use the last 10% of your disk space).
 
 Then run the ``petprep-docker`` command-line as if you were running
 ``petprep`` on a *bare-metal* installation::