deep-dive: Fix broken link of Percolator (#356)

Signed-off-by: Ping Yu <[email protected]>

Author: pingyu

content/deep-dive/distributed-transaction/percolator.md

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-TiKV supports distributed transactions, which is inspired by Google's [Percolator](https://research.google/pubs/pub36726.pdf). In this section, we will briefly introduce Percolator and how we make use of it in TiKV.
+TiKV supports distributed transactions, which is inspired by Google's [Percolator](https://research.google/pubs/large-scale-incremental-processing-using-distributed-transactions-and-notifications/). In this section, we will briefly introduce Percolator and how we make use of it in TiKV.
 
 ## What is Percolator?
 
-*Percolator* is a system built by Google for incremental processing on a very large data set. Since this is just a brief introduction, you can view the full paper [here](https://ai.google/research/pubs/pub36726#) for more details. If you are already very familiar with it, you can skip this section and go directly to [Percolator in TiKV](#percolator-in-tikv)
+*Percolator* is a system built by Google for incremental processing on a very large data set. Since this is just a brief introduction, you can view the full paper [here](https://www.usenix.org/legacy/event/osdi10/tech/full_papers/Peng.pdf) for more details. If you are already very familiar with it, you can skip this section and go directly to [Percolator in TiKV](#percolator-in-tikv)
 
 Percolator is built based on Google's BigTable, a distributed storage system that supports single-row transactions. Percolator implements distributed transactions in ACID snapshot-isolation semantics, which is not supported by BigTable. A column `c` of Percolator is actually divided into the following internal columns of BigTable: