Replace master references

Author: oschwald

README.md

@@ -170,7 +170,7 @@ thrown when querying the database.
 
 The MaxMind DB format is an open format for quickly mapping IP addresses to
 records. The
-[specification](https://github.com/maxmind/MaxMind-DB/blob/master/MaxMind-DB-spec.md)
+[specification](https://github.com/maxmind/MaxMind-DB/blob/main/MaxMind-DB-spec.md)
 is available, as is our
 [Perl writer](https://github.com/maxmind/MaxMind-DB-Writer-perl) for the
 format.