doc: fix description of the max header size in `HPE_HEADER_OVERFLOW` section
In the error documentation, the maximum header size was described with the old value of 8 KiB
, which has been corrected.
Here, the notation has been changed to maxHeaderSize
instead of the latest value of 16 KiB
, as the value could be overwritten by the --max-http-header-size
option.