Fixed more wrong directory names.

mcell_tutorial was inadvertently pluralized to mcell_tutorials in a
couple of places.
Clarified the variable reaction rate tutorial.

There were some potentially confusing parts to the variable reaction
rate tutorial. Also, it needed updated to work with CellBlender 0.1.57.
Instructions for those without root privileges.

The installation instructions were never very specific about making
MCell available from the user's PATH. Hopefully, this clarifies matters
for users with limited knowledge of such things, especially those
without root privileges.