Update README.md

Fixing the broken link to the design doc in the README.md

README.md

@@ -33,7 +33,7 @@ Because DM stores its state in the cluster, not on your workstation, you can ask
 those questions from any client at any time.
 
 For more information about types, including both primitive types and templates,
-see the [design document](../design/design.md#types).
+see the [design document](docs/design/design.md#types).
 
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