fix link to Prometheus documentation

exporter/cortexexporter/DESIGN.md

@@ -23,7 +23,7 @@ Cortex has a Remote Write API that accepts incoming metrics. This exporter shoul
 TimeSeries stores its metric name in its labels and does not describe metric types or start timestamps. To convert to TimeSeries data, buckets of a Histogram are broken down into individual TimeSeries with a bound label(`le`), and a similar process happens with quantiles in a Summary.
 
 
-More details of Remote Write API can be found in Prometheus [documentation](https://cortexmetrics.io/docs/apis/) and Cortex [documentation](https://cortexmetrics.io/docs/apis/).
+More details of Remote Write API can be found in Prometheus [documentation](https://prometheus.io/docs/prometheus/latest/storage/#overview) and Cortex [documentation](https://cortexmetrics.io/docs/apis/).
 
 ### **1.2 Gaps and Assumptions**