docs: Correct missing "d_" prefix for dentry_operations member d_weak_revalidate

The details for struct dentry_operations member d_weak_revalidate is
missing a "d_" prefix.

Fixes: af96c1e ("docs: filesystems: vfs: Convert vfs.txt to RST")
Signed-off-by: Glenn Washburn <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: crass

Documentation/filesystems/vfs.rst

@@ -1222,7 +1222,7 @@ defined:
 	return
 	-ECHILD and it will be called again in ref-walk mode.
 
-``_weak_revalidate``
+``d_weak_revalidate``
 	called when the VFS needs to revalidate a "jumped" dentry.  This
 	is called when a path-walk ends at dentry that was not acquired
 	by doing a lookup in the parent directory.  This includes "/",