[Fhsst-physics] Minor changes

[Vanessa McBride]

Hi Everyone

Just to let you know I've done a bit of editing to the chapters:
vectors.tex
forces.tex
rectilinear_motion.tex

All I've done is change the \step command in the Worked Examples to a more 
detailed comment on what
the step is about (\westep).  Please send any comments if you come across 
mistakes or have
suggestions.

Take care,
Vanessa