README: fix a type error in Offseting Paths example

Before this commit the `t = k / steps` math is done with integers,
which always results in `t = 0`.

This commit forces the math to be done with floating-point numbers,
which results in the progression of `t` values we want.

README.rst

@@ -587,7 +587,7 @@ curve <https://en.wikipedia.org/wiki/Parallel_curve>`__ for a few paths.
         for seg in path:
             ct = 1
             for k in range(steps):
-                t = k / steps
+                t = k / float(steps)
                 offset_vector = offset_distance * seg.normal(t)
                 nl = Line(seg.point(t), seg.point(t) + offset_vector)
                 nls.append(nl)