doc: corrects number of datasource used by pkgsite frontend

Change-Id: I20179fd648fc842e679a434f794c4ec8d4b489d2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/443616
TryBot-Result: kokoro <noreply+kokoro@google.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Dylan Le <dungtuanle@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/doc/frontend.md b/doc/frontend.md
index 9f838fe..c470ab8 100644
--- a/doc/frontend.md
+++ b/doc/frontend.md
@@ -21,7 +21,7 @@
 - The `-dev` flag reloads templates on each page load and rebuilds JavaScript
   assets when TypeScript source files change.
 
-The frontend can use one of three datasources:
+The frontend can use one of two datasources:
 
 - Postgres database
 - Proxy service