Fix OpenTelemetry.Tracer.end_span return type spec

[RudolfMan]

Hi there!

Based on the specs of otel_span:end_span/0,1 this Tracer.end_span/0,1 is supposed to always return span context (:undefined is not an option for the return)

See here:

opentelemetry-erlang/apps/opentelemetry_api/src/otel_span.erl

Lines 291 to 306 in 0a38bd4

	-spec end_span(SpanCtx) -> SpanCtx when
	SpanCtx :: opentelemetry:span_ctx().
	end_span(SpanCtx=#span_ctx{span_sdk={Module, _}}) when ?is_recording(SpanCtx) ->
	_ = Module:end_span(SpanCtx, undefined),
	SpanCtx#span_ctx{is_recording=false};
	end_span(SpanCtx) ->
	SpanCtx.
	%% @doc Ends the given span context with the given timestamp.
	%%
	%% If `SpanCtx' is not recording, this function doesn't do anything.
	%% If `Timestamp' is `undefined', this is equivalent to {@link end_span/1}.
	%% Returns the updated span context.
	-spec end_span(SpanCtx, Timestamp) -> SpanCtx when
	SpanCtx :: opentelemetry:span_ctx(),
	Timestamp :: integer() | undefined.

[tsloughter]

I believe the otel_tracer.erl may be the incorrect one as SpanCtx could be undefined. Not positive yet, but that is my guess here.