examples/web/README: fix 404s to opentelemetry-web (#664)
examples/web/README: fix 404s to opentelemetry-web (#664)

Closes #623
open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js-contrib