doc: add missing 'install' to 'apt-get' command

Author: coolaj86

doc/build-unix.md

@@ -74,7 +74,7 @@ Build requirements:
 
 Now, you can either build from self-compiled [depends](/depends/README.md) or install the required dependencies:
 
-    sudo apt-get libevent-dev libboost-system-dev libboost-filesystem-dev libboost-test-dev
+    sudo apt-get install libevent-dev libboost-system-dev libboost-filesystem-dev libboost-test-dev
 
 Berkeley DB is required for the wallet.