simple wording fix

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/parallel-plans.html
Description:

Hi!

In the first paragraph of the second part of the third sentence, this: 

"it must constructed so that each process which executes the plan" 

should be:

"it must be constructed so that each process which executes the plan".

That is, the "be" is missing.