Fix array attribute examples

[lmolkova]

Examples of array attributes are ambiguous, this PR disambiguates them.

E.g. it's unclear what this example means:

type: string[]
examples: [foo, bar]

Should it result in 2 examples ([foo] and [bar]) or one example ([foo, bar])?

Thanks to @jerbly for the context on how it should be interpreted:

The syntax document in build-tools does call out specifically: "A single example value for an array of strings attribute" examples: ['first element of first array', 'second element of first array'] as valid.

But it goes against yaml array syntax - the above example is equivalent to

type: string[]
examples: 
  - foo # this should be an array, not a string
  - bar # this should be an array, not a string

which then fails type checks being introduced in open-telemetry/weaver#300.

The proper syntax should be

type: string[]
examples: 
  - [foo] 
  - [foo, bar]

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

TIL it's possible to write

type: string[]
examples: 
  -
    - foo
  - 
    - foo
    - bar

🙈