Fix Style/FormatString offenses

`Kernel#format` is faster and will avoid to allocate an array compared
to `String#%`.

```
IPS:
       kernel_format:  3877614.2 i/s
      string_percent:  3531475.0 i/s - 1.10x  (± 0.00) slower

Memory:
       kernel_format:        160 allocated
      string_percent:        200 allocated - 1.25x more
```

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/axlsx/drawing/pic.rb b/lib/axlsx/drawing/pic.rb
index 150d6dc5..c9efb378 100644
--- a/lib/axlsx/drawing/pic.rb
+++ b/lib/axlsx/drawing/pic.rb
@@ -124,7 +124,7 @@ module Axlsx
     # The part name for this image used in serialization and relationship building
     # @return [String]
     def pn
-      "#{IMAGE_PN % [(index + 1), extname]}"
+      format(IMAGE_PN, index + 1, extname)
     end
 
     # The relationship object for this pic.