doc: add chrome version to browserless/chrome command

Change-Id: I918f2dc3aeae1247c9dc9adb1c314d978272b36a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/320411
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

doc/frontend.md

@@ -69,7 +69,7 @@ The e2e tests require that npm and docker are installed on your machine.
 
 First run headless chrome
 
-    docker run --rm -e "CONNECTION_TIMEOUT=-1" -p 3000:3000 browserless/chrome
+    docker run --rm -e "CONNECTION_TIMEOUT=-1" -p 3000:3000 browserless/chrome:1.46-chrome-stable
 
 Then run the tests