Add mc undo command to minio docs

[bh4t]

Describe the bug Description and example for undo command is missing under docs.min.io.

Here is the summary from terminal:

NAME: mc undo - undo PUT/DELETE operations

USAGE: mc undo [FLAGS] TARGET

FLAGS: --last value undo N last changes (default: 1) --recursive, -r undo last S3 PUT/DELETE operations recursively --force force recursive operation --dry-run fake an undo operation --config-dir value, -C value path to configuration folder (default: "/home/minio/.mc") --quiet, -q disable progress bar display --no-color disable color theme --json enable JSON lines formatted output --debug enable debug output --insecure disable SSL certificate verification --help, -h show help

EXAMPLES:

1. Undo the last 3 uploads and/or removals of a particular object $ mc undo s3/backups/file.zip --last 3

2. Undo the last upload/removal change of all objects under a prefix $ mc undo s3/backups/prefix/ --recursive --force