Use git package instead of obsolete git-core

`git-core` is obsolete on both Ubuntu 10.04 and 12.04.  We should be
using the `git` package.

Change-Id: I87fd52cf1947d8101eaa2a90d8625f806a5926be
diff --git a/src/source/initializing.md b/src/source/initializing.md
index ed98b99..6011fb8 100644
--- a/src/source/initializing.md
+++ b/src/source/initializing.md
@@ -90,7 +90,7 @@
 Building using a newer version of Ubuntu is currently only experimentally
 supported and is not guaranteed to work on branches other than master.
 
-    $ sudo apt-get install git-core gnupg flex bison gperf build-essential \
+    $ sudo apt-get install git gnupg flex bison gperf build-essential \
       zip curl zlib1g-dev libc6-dev lib32ncurses5-dev ia32-libs \
       x11proto-core-dev libx11-dev lib32readline5-dev lib32z-dev \
       libgl1-mesa-dev g++-multilib mingw32 tofrodos python-markdown \
@@ -109,7 +109,7 @@
 Building on Ubuntu 12.04 is currently only experimentally supported and is not
 guaranteed to work on branches other than master.
 
-    $ sudo apt-get install git-core gnupg flex bison gperf build-essential \
+    $ sudo apt-get install git gnupg flex bison gperf build-essential \
       zip curl libc6-dev libncurses5-dev:i386 x11proto-core-dev \
       libx11-dev:i386 libreadline6-dev:i386 libgl1-mesa-glx:i386 \
       libgl1-mesa-dev g++-multilib mingw32 openjdk-6-jdk tofrodos \