Fix broken doc-link from getting-started
This appears to be the only one.  Manual audit with (zsh):
pcregrep -o1 '}}/([^\)]+)\)' **/*(.)
yielded all the site-based URLs and I eye-compared them to the names of
existing docs.
@@ -45,4 +45,4 @@ utility that is included in the DNSControl repo (it converts
BIND-style zone files to DNSControl's language). To make sure the
migration is correct, use `dnscontrol preview` and make sure it
sees no differences.
-[Migration]({{site.github.url}}/migration) doc gives more detailed advice.
+[Migrating]({{site.github.url}}/migrating) doc gives more detailed advice.
