[DOCS] Small patch for the section "rules-views"

Hello,

While reading the documentation I spotted a minor glitch in the section 
"rules-views"

The reference at the bottom stating "That is the topic of the next 
section." is incorrect since the materialized views documentation got 
inserted between the section "rules-views" and "rules-update".

Attached is a small patch that fixes it by linking to the relevant 
section. (the patch should be applicable with "patch -p1" from the root 
dir of the postgres repo)

Regards,

Marc