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/drawing/num_val.rb b/lib/axlsx/drawing/num_val.rb
index 862d22ae..a945fb65 100644
--- a/lib/axlsx/drawing/num_val.rb
+++ b/lib/axlsx/drawing/num_val.rb
@@ -18,13 +18,13 @@ module Axlsx
 
     # @see format_code
     def format_code=(v)
-      Axlsx::validate_string(v)
+      Axlsx.validate_string(v)
       @format_code = v
     end
 
     # serialize the object
     def to_xml_string(idx, str = +'')
-      Axlsx::validate_unsigned_int(idx)
+      Axlsx.validate_unsigned_int(idx)
       unless v.to_s.empty?
         str << '<c:pt idx="' << idx.to_s << '" formatCode="' << format_code << '"><c:v>' << v.to_s << '</c:v></c:pt>'
       end