pstore/blk: update the command line example

Use the human readable device name instead of the device number, and
add the required best_effort parameter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: WeiXiong Liao <gmpy.liaowx@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201016132047.3068029-3-hch@lst.de

Documentation/admin-guide/pstore-blk.rst

@@ -35,7 +35,7 @@ module parameters have priority over Kconfig.
 
 Here is an example for module parameters::
 
-        pstore_blk.blkdev=179:7 pstore_blk.kmsg_size=64
+        pstore_blk.blkdev=/dev/mmcblk0p7 pstore_blk.kmsg_size=64 best_effort=y
 
 The detail of each configurations may be of interest to you.