FS-Cache: Fix heading in documentation

Fix a heading in the documentation to make it consistent with the contents
list.

Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/filesystems/caching/netfs-api.txt

@@ -433,7 +433,7 @@ to the caller.  The attribute adjustment excludes read and write operations.
 
 
 =====================
-PAGE READ/ALLOC/WRITE
+PAGE ALLOC/READ/WRITE
 =====================
 
 And the sixth step is to store and retrieve pages in the cache.  There are