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
```

3 files changed, 3 insertions, 3 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.
diff --git a/lib/axlsx/drawing/view_3D.rb b/lib/axlsx/drawing/view_3D.rb
index 9b3489a3..5b29b296 100644
--- a/lib/axlsx/drawing/view_3D.rb
+++ b/lib/axlsx/drawing/view_3D.rb
@@ -111,7 +111,7 @@ module Axlsx
       val = Axlsx.instance_values_for(self)[name]
       return "" if val.nil?
 
-      "<%s:%s val='%s'/>" % [namespace, Axlsx::camel(name, false), val]
+      format("<%s:%s val='%s'/>", namespace, Axlsx::camel(name, false), val)
     end
   end
 end
diff --git a/lib/axlsx/drawing/vml_drawing.rb b/lib/axlsx/drawing/vml_drawing.rb
index 364ce19a..15698486 100644
--- a/lib/axlsx/drawing/vml_drawing.rb
+++ b/lib/axlsx/drawing/vml_drawing.rb
@@ -14,7 +14,7 @@ module Axlsx
     # The part name for this vml drawing
     # @return [String]
     def pn
-      "#{VML_DRAWING_PN}" % (@comments.worksheet.index + 1)
+      VML_DRAWING_PN % (@comments.worksheet.index + 1)
     end
 
     # serialize the vml_drawing to xml.