Update docs/installation.rst

Co-authored-by: Andrew Murray <3112309+radarhere@users.noreply.github.com>

Author: andreasnuesslein

docs/installation.rst

@@ -328,7 +328,7 @@ In Fedora, the command is::
 
 .. Note:: ``redhat-rpm-config`` is required on Fedora 23, but not earlier versions.
 
-Prerequisites for **Ubuntu 16.04 LTS or later** are installed with::
+Prerequisites for **Ubuntu 16.04 LTS - 20.04 LTS** are installed with::
 
     sudo apt-get install libtiff5-dev libjpeg8-dev libopenjp2-7-dev zlib1g-dev \
         libfreetype6-dev liblcms2-dev libwebp-dev tcl8.6-dev tk8.6-dev python3-tk \