Fixed broken link: Containers/C++ B-tree

Previous link was pointing to a code.google.com repository, which
reported a 404 error.

New link seems to be not official, but working ;)

README.md

@@ -300,7 +300,7 @@ A curated list of awesome C++ (or C) frameworks, libraries, resources, and shiny
 
 ## Containers
 
-* [C++ B-tree](https://code.google.com/p/cpp-btree/) - A template library that implements ordered in-memory containers based on a B-tree data structure. [Apache2]
+* [C++ B-tree](https://github.com/algorithm-ninja/cpp-btree) - A template library that implements ordered in-memory containers based on a B-tree data structure. [Apache2]
 * [Colony](https://github.com/mattreecebentley/plf_colony) - An unordered "bag"-type container which outperforms std containers in high-modification scenarios while maintaining permanent pointers to non-erased elements regardless of insertion/erasure. [zLib] [website](http://www.plflib.org/colony.htm)
 * [dynamic_bitset](https://github.com/pinam45/dynamic_bitset) - A C++17 header-only dynamic bitset. [MIT]
 * [Forest](https://github.com/xorz57/forest) - Template library implementing an AVL, a Binary Search, a KD and a Quad Tree. [MIT]