Fix docs for checking out Skia

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2179023003
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2179023003

Review-Url: https://codereview.chromium.org/2179023003
diff --git a/site/user/download.md b/site/user/download.md
index 831d301..5da01a2 100644
--- a/site/user/download.md
+++ b/site/user/download.md
@@ -21,7 +21,10 @@
 
 <!--?prettify lang=sh?-->
 
-    git clone https://skia.googlesource.com/skia.git
+    mkdir $SRC
+    cd $SRC
+    gclient config --unmanaged https://skia.googlesource.com/skia.git
+    gclient sync
     cd skia
 
 Getting started with Skia