fix: otlp.md missing closing details label. (#1278)
fix: otlp.md missing closing details label. (#1278)

## Changes

It seems wired that the closing tag of `<details>` is missing. As a result, most of the content has been folded. 
Added a closing `</details>` tags to only fold the table of contents.

Before the change:
![image](https://user-images.githubusercontent.com/12531298/101312037-ccbd5080-3820-11eb-89d0-e153de96dacd.png)

After the change:
![image](https://user-images.githubusercontent.com/12531298/101312107-ef4f6980-3820-11eb-86c9-f8a35627dc9b.png)

open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification