doc: fix formatting in frontend doc

Change-Id: I00406b65b2488c004cf3307b5a58edec3f1dba48
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/280894
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/doc/frontend.md b/doc/frontend.md
index c0517f7..97fc9cc 100644
--- a/doc/frontend.md
+++ b/doc/frontend.md
@@ -58,6 +58,7 @@
 
 To run the tests locally, start the pkgsite server and then run
 `./devtools/npm.sh run test-e2e`
+
 ### Compiling JS sources
 
 The frontend serves compiled and minified JS. If you're modifying any JS code, make