Fix the version number in the changelog.

I changed the pubspec but I forgot this.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1168923002.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 96df1dd..123c564 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 0.13.1+1
+## 0.13.2
 
 * Allow option values that look like options. This more closely matches the
   behavior of [`getopt`][getopt], the *de facto* standard for option parsing.