Docs: ooops, debug is the default in GN.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2195173002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2195173002

TBR=jcgregorio@google.com

Review-Url: https://codereview.chromium.org/2195173002
diff --git a/site/user/quick/gn.md b/site/user/quick/gn.md
index 93f7680..c710576 100644
--- a/site/user/quick/gn.md
+++ b/site/user/quick/gn.md
@@ -28,8 +28,8 @@
     gclient sync && bin/fetch-gn
 
     # Run GN to generate your build files.  Some examples.
-    gn gen out/Release
-    gn gen out/Debug --args=is_debug=true
+    gn gen out/Debug
+    gn gen out/Release --args=is_debug=false
     gn gen out/Clang --args='cc="clang" cxx="clang++"'
     gn gen out/Shared --args=is_component_build=true