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: 46c4c9d1be ("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

Documentation/admin-guide/sysctl/fs.rst

@@ -164,8 +164,8 @@ pipe-user-pages-soft
 --------------------
 
 Maximum total number of pages a non-privileged user may allocate for pipes
-before the pipe size gets limited to a single page. Once this limit is reached,
-new pipes will be limited to a single page in size for this user in order to
+before the pipe size gets limited to two pages. Once this limit is reached,
+new pipes will be limited to two pages in size for this user in order to
 limit total memory usage, and trying to increase them using ``fcntl()`` will be
 denied until usage goes below the limit again. The default value allows to
 allocate up to 1024 pipes at their default size. When set to 0, no limit is