Fixed a deprecation warning on the pyplot backend

The follwing code created a deprecation warining:
using Plots; pyplot()
plot([1,2,3,4], [2,3,4,5], yscale=:log10)

src/backends/pyplot.jl

@@ -926,7 +926,7 @@ function py_compute_axis_minval(axis::Axis)
         for series in series_list(sp)
             v = series.d[axis[:letter]]
             if !isempty(v)
-                minval = NaNMath.min(minval, ignorenan_minimum(abs(v)))
+                minval = NaNMath.min(minval, ignorenan_minimum(abs.(v)))
             end
         end
     end