doc: Bluetooth: Mesh: Fix line width to 100 in mesh docs

From Codying Style guideline:
https://docs.zephyrproject.org/latest/contribute/guidelines.html#coding-style



`The line length is 100 columns or fewer. In the documentation, longer
lines for URL references are an allowed exception.`

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>