docs: iio: mark "repeat" sysfs attribute as optional

Show that this field is optional, just like the shift value.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-6-luca@lucaceresoli.netSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/driver-api/iio/buffers.rst

@@ -37,7 +37,7 @@ attributes of the following form:
 * :file:`index`, the scan_index of the channel.
 * :file:`type`, description of the scan element data storage within the buffer
   and hence the form in which it is read from user space.
-  Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] .
+  Format is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift] .
 
   * *be* or *le*, specifies big or little endian.
   * *s* or *u*, specifies if signed (2's complement) or unsigned.