Update installation.md

Add the -y option to the python apt-get install commands to be consistent with the rest of the Document.

doc/install/installation.md

@@ -63,7 +63,7 @@ Make sure you have the right version of Python installed.
     python --version
     # If it's Python 3 you might need to install Python 2 separately
-    sudo apt-get install python2.7
+    sudo apt-get install -y python2.7
     # Make sure you can access Python via python2
     python2 --version
@@ -72,7 +72,7 @@ Make sure you have the right version of Python installed.
     sudo ln -s /usr/bin/python /usr/bin/python2
     # For reStructuredText markup language support install required package:
-    sudo apt-get install python-docutils
+    sudo apt-get install -y python-docutils
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