design/25530-sumdb: fix typo

Change-Id: Ib2ab9fb665f93fafbcf18d22c3080b6f8df984c8
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/193657
Reviewed-by: Filippo Valsorda <filippo@golang.org>

design/25530-sumdb.md

diff --git a/design/25530-sumdb.md b/design/25530-sumdb.md
index f6ee726..8d072e1 100644
--- a/design/25530-sumdb.md
+++ b/design/25530-sumdb.md
@@ -233,7 +233,7 @@
 only modules and not checksum data—in particular,
 module-only proxies can be served from entirely static file systems,
 with no special infrastructure at all.
-Such proxies can respoond with an HTTP 404 or HTTP 410 to
+Such proxies can respond with an HTTP 404 or HTTP 410 to
 the `/sumdb/supported` endpoint, so that clients
 will connect to the database directly.