fix(bindings/erlang): correct ex_doc source links for monorepo layout
ex_doc defaulted per-function source links to blob/v1.0.0/<path>, which
404s twice over: the real git tag is monorepo-scoped (erlang-v1.0.0), and
the sources live under bindings/erlang/, not the repo root. Add a
source_url_pattern that bakes in both the scoped tag and the subdirectory.