Fix safe Style/ColonMethodCall offenses

Caxlsx is using both `.` and `::`, 220 occurrences vs 280 to invoke
methods on `Axlsx` module.

This commit standardizes the approach towards `.`, which will also allow
shorter lines.

Performance is not affected

```
Comparison:
      Axlsx.validate:  8515252.3 i/s
     Axlsx::validate:  8512863.7 i/s - same-ish: difference falls within error 
```

1 files changed, 3 insertions, 3 deletions

diff --git a/lib/axlsx/drawing/area_series.rb b/lib/axlsx/drawing/area_series.rb
index 30bd9402..0e3d8928 100644
--- a/lib/axlsx/drawing/area_series.rb
+++ b/lib/axlsx/drawing/area_series.rb
@@ -52,19 +52,19 @@ module Axlsx
 
     # @see show_marker
     def show_marker=(v)
-      Axlsx::validate_boolean(v)
+      Axlsx.validate_boolean(v)
       @show_marker = v
     end
 
     # @see marker_symbol
     def marker_symbol=(v)
-      Axlsx::validate_marker_symbol(v)
+      Axlsx.validate_marker_symbol(v)
       @marker_symbol = v
     end
 
     # @see smooth
     def smooth=(v)
-      Axlsx::validate_boolean(v)
+      Axlsx.validate_boolean(v)
       @smooth = v
     end