fix add_event macro to call otel_span:add_event/3

[tsloughter]

Fixes #144

[codecov]

Codecov Report

Merging #145 (e9576bc) into master (1d5a5a0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   32.27%   32.27%           
=======================================
  Files          62       62           
  Lines        3145     3145           
=======================================
  Hits         1015     1015           
  Misses       2130     2130           

Flag	Coverage Δ
api	50.22% <ø> (+0.22%)	⬆️
elixir	14.28% <ø> (ø)
erlang	32.13% <ø> (ø)
exporter	18.02% <ø> (ø)
sdk	66.88% <ø> (-0.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_batch_processor.erl	72.72% <0.00%> (-1.14%)	⬇️
apps/opentelemetry_api/src/otel_span.erl	68.42% <0.00%> (+5.26%)	⬆️

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