Update opentelemetry-specification

[chalin]

Followup to #715, bringing the schemas back in line with the changes introduced by #695.

This PR was created by running the following command:

$ npm run submodule:update content-modules/opentelemetry-specification

Preview tests:

• https://deploy-preview-719--opentelemetry.netlify.app/schemas/1.6.1 still exists
• https://deploy-preview-719--opentelemetry.netlify.app/schemas/1.6.0 is gone

/cc @tigrannajaryan

[chalin]

This PR was created by running the following command:

$ npm run submodule:update content-modules/opentelemetry-specification

@austinlparker - we had discussed the possibility of having a single command to generate the PR from the command line. Well, this is it 😄

[chalin]

If we want something simpler, we can always add another script named, say update-spec so that the command would become:

$ npm run update-spec

[chalin]

Since this is a simple PR bringing the site back to a state that it was recently, I'm going to merge now.

[chalin]

If we want something simpler, we can always add another script named, say update-spec so that the command would become:

$ npm run update-spec

FYI, #725 adds an npm script named schemas:update.