Update build.md
Don't specify building from testnet3 since it doesn't include necessary commits
@@ -44,7 +44,6 @@ If you decide to use a persistent storage (e.g. ```-v grin:/usr/src/grin```) you
```sh
git clone https://github.com/mimblewimble/grin.git
cd grin
-git checkout milestone/testnet3
cargo build --release
```
