fix typos in the NOTES.md and README.md files, add dependency for compiling VTK

[pr0m1th3as]

in NOTES.md, the required lib is misspelled. it should be: libfltk1.3-dev
in README.md, the command to untar VTK should be: tar -xvf VTK-8.2.0.tar.gz

compiling VTK also requires "libqt5x11extras5-dev" package to be installed otherwise cmake returns errors.
I suggest appending the package in the necessary build tools...
sudo apt install build-essential qtbase5-dev "libqt5x11extras5-dev cmake-curses-gui