docs/registration: replace \triangleq -> \equiv

The ``\triangleq`` command is an AMS symbol and the PDF documentation is not set up to use the ``amssymb`` just yet. It doesn't really matter if ``\triangleq`` or ``\equiv`` are used.
Merge pull request #70 from rjw57/fix-rtd-docs-build

docs/registration: replace \triangleq -> \equiv