The section about custom pprof paths referenced the wrong path.

This also fixes a couple minor grammatical issues elsewhere in the doc.

Fixes #22832

Change-Id: I890cceb53a13c1958d9cf958c658ccfcbb6863d5
Reviewed-on: https://go-review.googlesource.com/79035Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>