Fix Style/RedundantInterpolation offenses

`%` is an operation on `String` that will return a new `String`, so
the interpolation is redundant

Also adds a missing spec on PivotTable#rels_pn

```
IPS:
      uninterpolated:  4045715.7 i/s
        interpolated:  2359775.6 i/s - 1.71x  (± 0.00) slower

Memory:
      uninterpolated:        160 allocated
        interpolated:        232 allocated - 1.45x more
```

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/axlsx/drawing/chart.rb b/lib/axlsx/drawing/chart.rb
index 9f59dc6a..6b3acab1 100644
--- a/lib/axlsx/drawing/chart.rb
+++ b/lib/axlsx/drawing/chart.rb
@@ -124,7 +124,7 @@ module Axlsx
     # The part name for this chart
     # @return [String]
     def pn
-      "#{CHART_PN % (index + 1)}"
+      CHART_PN % (index + 1)
     end
 
     # The title object for the chart.