_content/learn: point doc card to /doc/

Fixes golang/go#49971

Change-Id: I37bdec79680963c3cbcc87ab0ff17ee4a3c4a011
Reviewed-on: https://go-review.googlesource.com/c/website/+/370434
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Trust: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jamal Carvalho <jamalcarvalho@google.com>
diff --git a/_content/learn/quickstart.yaml b/_content/learn/quickstart.yaml
index d396cc2..928be0b 100644
--- a/_content/learn/quickstart.yaml
+++ b/_content/learn/quickstart.yaml
@@ -2,7 +2,7 @@
     content: Everything there is to know about Go. Get started on a new project
       or brush up for your existing Go code.
     cta: View documentation
-    url: /doc/install
+    url: /doc/
   - title: Tour of Go
     content: An interactive introduction to Go in three sections. Each section
       concludes with a few exercises so you can practice what you've learned.