Ubuntu build HOWTO: Suggest to install git instead of git-core

The package docs for the git-core Ubuntu package in all supported
Ubuntu versions, including 12.04 say:

This is a transitional dummy package.  The 'git-core' package has been
renamed to 'git', which has been installed automatically.  This git-core
package is now obsolete, and can safely be removed from the system if no
other package depends on it.

Fixes #4109

README.txt

@@ -111,7 +111,7 @@ $ sudo dnf install make automake gcc gcc-c++ kernel-devel cmake libcurl* openal*
 You can install git for easily keeping your copy up to date.
 If you dont want git, read below on how to get the source without git.
 This is an example for installing git on Debian/Ubuntu:
-$ sudo apt-get install git-core
+$ sudo apt-get install git
 
 For Fedora users:
 $ sudo dnf install git-core