Make true/false values lowercase in Metadata Schema specification

[npatki]

Boolean values in JSON are lowercase (true/false) even though they are uppercase in Python (True/False).

The User Guide for the Metadata Schema says it's describing JSON files at the top. However, it's including an uppercase boolean value as an example.

"social_security_number": {
    "type": "categorical",
    "pii": True,
    "pii_category": "ssn"
}

This might confuse users if they're following the guide to write their metadata.json.