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/rels/relationship.rb b/lib/axlsx/rels/relationship.rb
index 93cdc88b..62860f50 100644
--- a/lib/axlsx/rels/relationship.rb
+++ b/lib/axlsx/rels/relationship.rb
@@ -92,9 +92,9 @@ module Axlsx
     end
 
     # @see Target
-    def Target=(v) Axlsx::validate_string v; @Target = v end
+    def Target=(v) Axlsx.validate_string v; @Target = v end
     # @see Type
-    def Type=(v) Axlsx::validate_relationship_type v; @Type = v end
+    def Type=(v) Axlsx.validate_relationship_type v; @Type = v end
 
     # @see TargetMode
     def TargetMode=(v) RestrictionValidator.validate 'Relationship.TargetMode', [:External, :Internal], v; @TargetMode = v; end
@@ -107,7 +107,7 @@ module Axlsx
       str << '<Relationship '
       h.each_with_index do |key_value, index|
         str << ' ' unless index.zero?
-        str << key_value.first.to_s << '="' << Axlsx::coder.encode(key_value.last.to_s) << '"'
+        str << key_value.first.to_s << '="' << Axlsx.coder.encode(key_value.last.to_s) << '"'
       end
       str << '/>'
     end