Replace master references

Author: oschwald

README.rst

@@ -33,7 +33,7 @@ Database Reader Extension
 If you wish to use the C extension for the database reader, you must first
 install the `libmaxminddb C API <https://github.com/maxmind/libmaxminddb>`_.
 Please `see the instructions distributed with it
-<https://github.com/maxmind/libmaxminddb/blob/master/README.md>`_.
+<https://github.com/maxmind/libmaxminddb/blob/main/README.md>`_.
 
 IP Geolocation Usage
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