Update temporal graph in specification.md

[wu-sheng]

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Make Temporal relationships between Spans in a single Trace more sense.

Based on https://github.com/opentracing/specification/pull/103#discussion_r170787736

cc @christophetd

[yurishkuro]

I actually like the original better. It would be even better if there were causality indications, like an edge from A to both B and C. There's nothing wrong with both of them being the children of A and both starting at different times. This PR changes the graph to a more special case of sequential spans.

[wu-sheng]

This PR changes the graph to a more special case of sequential spans

The graph always visualized spans. I agree new graph is sequential spans case, but I think it is more common, but not so special. In my perspective, most language application runs in sequential way. The old graph shows more about a async process graph, which may confuse people, like #103

I agree either way is just a case, this is not a right/wrong discussion. I just want to make people more easier to understand when they read the document at first time. @yurishkuro

[tedsuo]

The real answer is to use an actual image of a graph, with edges and a legend. I don't really feel like this is better or worse.

[jpkrohling]

The real answer is to use an actual image of a graph

This.