HE: Fix he_bss_color documentation

HE: Fix he_bss_color documentation

This field needs to be set to a value within 1-63 range, i.e., 0 is not
a valid value and does not indicate that BSS color is disabled. B7 of
the BSS Color octet is used to indicate that the BSS Color is
_temporarily_ disabled, but that is something that would happen
automatically based on detecting a collision in the used BSS colors and
not something that would be configured.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>