duplicate links to manticore blogposts

[0xalpharush]

Links 3 and 4 link to the same blogpost as 2

building-secure-contracts/program-analysis/manticore/symbolic-execution-introduction.md

Line 3 in c7f4f09

[Manticore]() is a dynamic symbolic execution tool, we described in our previous blogposts ([1](https://blog.trailofbits.com/2017/04/27/manticore-symbolic-execution-for-humans/), [2](https://blog.trailofbits.com/2017/05/15/magic-with-manticore/), [3](https://blog.trailofbits.com/2017/05/15/magic-with-manticore/), [4](https://blog.trailofbits.com/2017/05/15/magic-with-manticore/)).