[DOC] Update OpenTracing shim README.md

[chalin]

• Followup to Opentracing shim #1909
• Adds a direct link to the Migrating from OpenTracing page from the OTel website (that page contains a link to the corresponding specification page).
• Makes a few copyedits
• Related: Update link to C++ shim opentelemetry.io#2466

Preview: https://github.com/open-telemetry/opentelemetry-cpp/blob/79ecce82bb29d2a71a66f8fd79c8d33c26b17ce9/opentracing-shim/README.md

/cc @chusitoo @lalitb @reyang

[marcalff]

LGTM, thanks for the doc updates.

[codecov]

Codecov Report

Merging #2028 (79ecce8) into main (f702676) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

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  Coverage   87.32%   87.32%           
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  Files         166      166           
  Lines        4723     4723           
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  Hits         4124     4124           
  Misses        599      599