Fix typos in CONTRIBUTORS.md: 'a open' -> 'an open' and 'Committers comes' -> 'Committers come'

[nakanoh]

Fix typos in CONTRIBUTORS.md

• Fix article usage: "a open source project" -> "an open source project"
  The indefinite article "a" should be "an" before a word starting with a vowel sound.
• Fix subject-verb agreement: "Committers comes" -> "Committers come"
  The plural subject "Committers" requires the plural form of the verb.

These changes improve the grammatical correctness of the documentation.

[hcho3]

Thanks!