docs: filesystems: overlayfs: Rename overlayfs.txt to .rst

It is already formatted as RST.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

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@@ -12220,7 +12220,7 @@ L:	linux-unionfs@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git
 S:	Supported
 F:	fs/overlayfs/
-F:	Documentation/filesystems/overlayfs.txt
+F:	Documentation/filesystems/overlayfs.rst
 
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 M:	Christian Lamparter <chunkeey@googlemail.com>