docs - corrected a clerical error about 'gp_resource_group_memory_limit'

[fairyfar]

There is a clerical error about 'gp_resource_group_memory_limit' introductions.

For example, 0.175 means 17.5%, so we should not add '%' after 0.175.

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• [ ] Add tests for the change
• [x] Document changes
• [ ] Communicate in the mailing list if needed
• [ ] Pass make installcheck
• [ ] Review a PR in return to support the community