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, 2 insertions, 2 deletions

diff --git a/lib/axlsx/package.rb b/lib/axlsx/package.rb
index 4597efc0..4f793d41 100644
--- a/lib/axlsx/package.rb
+++ b/lib/axlsx/package.rb
@@ -33,7 +33,7 @@ module Axlsx
     # Shortcut to specify that the workbook should use autowidth
     # @see Workbook#use_autowidth
     def use_autowidth=(v)
-      Axlsx::validate_boolean(v)
+      Axlsx.validate_boolean(v)
       workbook.use_autowidth = v
     end
 
@@ -46,7 +46,7 @@ module Axlsx
     # Shortcut to specify that the workbook should use shared strings
     # @see Workbook#use_shared_strings
     def use_shared_strings=(v)
-      Axlsx::validate_boolean(v)
+      Axlsx.validate_boolean(v)
       workbook.use_shared_strings = v
     end