25530-sumdb.md: fix a typo in the endpoint reference
25530-sumdb.md: fix a typo in the endpoint reference

Change-Id: Icbf807364f64585d23ca2e36b919aeb57a068f1b
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/189320
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/proposal,golang/proposal