fix(rds_instance_status): metric description and doc in readme

[qfritz]

No description provided.

[github-actions]

Package	Line Rate	Health
github.com/qonto/prometheus-rds-exporter/cmd	0%	❌
github.com/qonto/prometheus-rds-exporter/internal/app/cloudwatch/mock	100%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/cloudwatch	87%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/ec2	87%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/ec2/mock	51%	❌
github.com/qonto/prometheus-rds-exporter/internal/app/exporter	80%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/rds	67%	➖
github.com/qonto/prometheus-rds-exporter/internal/app/rds/mock	89%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/servicequotas/mock	47%	❌
github.com/qonto/prometheus-rds-exporter/internal/app/servicequotas	67%	➖
github.com/qonto/prometheus-rds-exporter/internal/app/trace	100%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/unit	100%	✔
github.com/qonto/prometheus-rds-exporter/internal/infra/http	0%	❌
github.com/qonto/prometheus-rds-exporter/internal/infra/logger	79%	➖
github.com/qonto/prometheus-rds-exporter/internal/infra/tracing	0%	❌
github.com/qonto/prometheus-rds-exporter	0%	❌
Summary	64% (1193 / 1856)	➖

Minimum allowed line rate is 60%

[dcupif]

LGTM 🚀