Revise netlify.toml example
This is extremely minor, but the initial `netlify.toml` example confused
me a bit because I didn't realize that it was using a *netlify*
variable; I thought I needed to replace something with a variable from
my own configuration.  I made a very minor edit to the docs to clarify.
@@ -31,11 +31,13 @@ command = "gutenberg build"
[build.environment]
# Set the version name that you want to use and Netlify will automatically use it
-GUTENBERG_VERSION = "0.3.1"
+GUTENBERG_VERSION = "0.3.3"
# The magic for deploying previews of branches
-# We need to override the base url with what the url of the preview is ($DEPLOY_PRIME_URL)
-# otherwise links would not work properly
+# We need to override the base url with whatever url Netlify assigns to our
+# preview site. We do this using the Netlify environment variable
+# `$DEPLOY_PRIME_URL`.
+
[context.deploy-preview]
command = "gutenberg build --base-url $DEPLOY_PRIME_URL"
