docs: Use vfs module instead of os.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/machine.SD.rst

@@ -20,11 +20,11 @@ more info regarding the pins which can be remapped to be used with a SD card.
 Example usage::
 
     from machine import SD
-    import os
+    import vfs
     # clk cmd and dat0 pins must be passed along with
     # their respective alternate functions
     sd = machine.SD(pins=('GP10', 'GP11', 'GP15'))
-    os.mount(sd, '/sd')
+    vfs.mount(sd, '/sd')
     # do normal file operations
 
 Constructors

docs/library/machine.SDCard.rst

@@ -30,7 +30,7 @@ vary from platform to platform.
     The class implements the block protocol defined by :class:`vfs.AbstractBlockDev`.
     This allows the mounting of an SD card to be as simple as::
 
-      os.mount(machine.SDCard(), "/sd")
+      vfs.mount(machine.SDCard(), "/sd")
 
     The constructor takes the following parameters:
 

docs/library/pyb.rst

@@ -213,7 +213,7 @@ Miscellaneous functions
 .. function:: mount(device, mountpoint, *, readonly=False, mkfs=False)
 
    .. note:: This function is deprecated. Mounting and unmounting devices should
-      be performed by :meth:`os.mount` and :meth:`os.umount` instead.
+      be performed by :meth:`vfs.mount` and :meth:`vfs.umount` instead.
 
    Mount a block device and make it available as part of the filesystem.
    ``device`` must be an object that provides the block protocol. (The