docs: filesystems: corrected grammar of netfs page

Fixed the word "aren't" to "isn't" based on singular word "bufferage".

Signed-off-by: Dennis Lam <dennis.lamerice@gmail.com>
Link: https://lore.kernel.org/r/20240912012550.13748-2-dennis.lamerice@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Dennis Lam

Documentation/filesystems/netfs_library.rst

@@ -116,7 +116,7 @@ The following services are provided:
  * Handle local caching, allowing cached data and server-read data to be
    interleaved for a single request.
 
- * Handle clearing of bufferage that aren't on the server.
+ * Handle clearing of bufferage that isn't on the server.
 
  * Handle retrying of reads that failed, switching reads from the cache to the
    server as necessary.