[readme] Fix install command and add suitesparse comment

dumux-course installs the dumux-course package including grid managers (not only the course module)

README.md

@@ -21,7 +21,7 @@ for instance, with this sequence of commands:
 ```bash
 wget https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/raw/releases/3.5/bin/installdumux.py
 python3 installdumux.py --dumux-version 3.5 --dune-version 2.8
-python3 dumux/bin/installexternal.py course
+python3 dumux/bin/installexternal.py dumux-course
 ./dune-common/bin/dunecontrol --opts=./dumux/cmake.opts all
 ```
 
@@ -35,7 +35,7 @@ navigate to your DuMu<sup>x</sup> root directory
 and then run the following commands:
 
 ```bash
-python3 dumux/bin/installexternal.py course
+python3 dumux/bin/installexternal.py dumux-course
 ./dune-common/bin/dunecontrol --opts=./dumux/cmake.opts all
 ```
 
@@ -55,5 +55,7 @@ software packages are needed:
   * gnuplot (to plot some curves)
   * wget (to download some config files during the installation)
 
+For some excercises you might need UMFPack. UMFPack can be installed via Suitesparse.
+For instance on Ubuntu use `apt-get install libsuitesparse-dev`.
 
 Last updated for: __DuMu<sup>x</sup> release 3.5 in May 2022__.