book/zkas: Make a link work.

doc/src/SUMMARY.md

@@ -15,7 +15,7 @@
   - [darkfid JSON-RPC API](clients/darkfid_jsonrpc.md)
   - [cashierd JSON-RPC API](clients/cashierd_jsonrpc.md)
   - [faucetd JSON-RPC API](clients/faucetd_jsonrpc.md)
-- [zkas](zkas/zkas.md)
+- [zkas](zkas/index.md)
   - [Bincode](zkas/bincode.md)
   - [Examples](zkas/examples.md)
     - [Sapling scheme](zkas/examples/sapling.md)

doc/src/zkas/index.md

@@ -0,0 +1 @@
+zkas.md