Update series.tex

Line 133 (page 42, near the bottom), changed 
&=a_0\, (1+r+r^2+r^3+\cdots+r^n) - a_0\, (r+r^2+r^3+\cdots+r^{n-1}+r^{n+1})
to
&=a_0\, (1+r+r^2+r^3+\cdots+r^n) - a_0\, (r+r^2+r^3+\cdots+r^n+r^{n+1})