mount_namespaces.7: Update references to Documentation/filesystems/sharedsubtree.rst

Documentation/filesystems/sharedsubtree.txt has changed to
Documentation/filesystems/sharedsubtree.rst.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/mount_namespaces.7

@@ -701,7 +701,7 @@ as for a bind operation on each mount in the subtree.
 (Unbindable mounts are automatically pruned at the target mount point.)
 .PP
 For further details, see
-.I Documentation/filesystems/sharedsubtree.txt
+.I Documentation/filesystems/sharedsubtree.rst
 in the kernel source tree.
 .\"
 .SS Move (MS_MOVE) semantics
@@ -743,7 +743,7 @@ dest(B)	shared	shared	shared	slave+shared	invalid
 Note: moving a mount that resides under a shared mount is invalid.
 .PP
 For further details, see
-.I Documentation/filesystems/sharedsubtree.txt
+.I Documentation/filesystems/sharedsubtree.rst
 in the kernel source tree.
 .\"
 .SS Mount semantics
@@ -1039,7 +1039,7 @@ For a discussion of propagation types when moving mounts
 and creating bind mounts
 .RB ( MS_BIND ),
 see
-.IR Documentation/filesystems/sharedsubtree.txt .
+.IR Documentation/filesystems/sharedsubtree.rst .
 .\"
 .\" ============================================================
 .\"
@@ -1346,5 +1346,5 @@ See
 .BR pivot_root (8),
 .BR umount (8)
 .PP
-.IR Documentation/filesystems/sharedsubtree.txt
+.IR Documentation/filesystems/sharedsubtree.rst
 in the kernel source tree.