broken commit url fixed

http://mikera.github.io/core.matrix/summary.html
Current url in the head leads to [404](https://github.com/mikera/core.matrix/blob/dc0f3d66ad7c48493f4110fc15621a7d0f817647)
With this fix it will lead to https://github.com/mikera/core.matrix/tree/dc0f3d66ad7c48493f4110fc15621a7d0f817647