docs: Update SnowflakeMetadataExtractor instructions in readme

[Alagappan]

Signed-off-by: Alagappan Sethuraman alagappan.als@gmail.com

Summary of Changes

Configs for Snowflake metadata extractor was referring postgres metadata extractor. fixed them to have snowflake ones

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[Alagappan]

cc @feng-tao This should be a quick one. instructions has wrong configs. Fixed them

[Alagappan]

@jornh