docs: admin-guide: update to current minimum pipe size default

docs: admin-guide: update to current minimum pipe size default

The pipe size limit used when the fs.pipe-user-pages-soft sysctl value
is reached was increased from one to two pages in commit 46c4c9d1beb7;
update the documentation to match the new reality.

Fixes: 46c4c9d1beb7 ("pipe: increase minimum default pipe size to 2 pages")
Signed-off-by: Štěpán Němec <stepnem@smrk.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250729-pipedoc-v2-1-18b8e735a9c6@smrk.net