doc: change line length limit in contributors guide

doc: change line length limit in contributors guide

The DPDK documentation guidelines state that lines should be wrapped as
follows:

* The recommended style for the DPDK documentation is to put sentences on
  separate lines. This allows for easier reviewing of patches. ...
* Long sentences should be wrapped at 120 characters +/- 10 characters.
  They should be wrapped at words.

  http://dpdk.org/doc/guides/contributing/documentation.html#line-length

However, in practice, users almost never submit doc patches in this format.
Instead most users wrap their doc patches at 80 characters.

This patch updates the documentation contributors guide to reflect this
as the recommended guideline.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Shreyansh Jain <shreyansh.jain@nxp.com>