doc:modfiy ubuntu build on 18.04

Signed-off-by: wenlingz <wenling.zhang@intel.com>

Author: wenlingz

doc/getting-started/building-from-source.rst

@@ -95,7 +95,7 @@ Install the necessary tools for the following systems:
 
   .. note::
      Use ``gcc`` version 7.3.* or higher to avoid running into
-     issue `#1396 <https://github.com/projectacrn/acrn-hypervisor/issues/1396>`_. Follow these instructions to install the ``gcc-7`` package on Ubuntu 16.04:
+     issue `#1396 <https://github.com/projectacrn/acrn-hypervisor/issues/1396>`_. Follow these instructions to install the ``gcc-7`` package on Ubuntu 18.04:
 
      .. code-block:: none
 
@@ -108,20 +108,8 @@ Install the necessary tools for the following systems:
 
      ACRN development requires ``binutils`` version 2.27 (or higher).
      Verify your version of ``binutils`` with the command ``apt show binutils
-     ``. While Ubuntu 18.04 has a new version of ``binutils``, the default
-     version on Ubuntu 16.04 must be updated (see issue `#1133
-     <https://github.com/projectacrn/acrn-hypervisor/issues/1133>`_).
+     ``.
 
-     .. code-block:: none
-
-        $ wget https://mirrors.ocf.berkeley.edu/gnu/binutils/binutils-2.27.tar.gz
-        $ tar xzvf binutils-2.27.tar.gz && cd binutils-2.27
-        $ ./configure
-        $ make
-        $ sudo make install
-
-
-     Ubuntu 14.04 requires ``libsystemd-journal-dev`` instead of ``libsystemd-dev`` as indicated above.
 
 * Fedora/Redhat development system: