docs: use as_json per method instead of to_json

The docs usage should align with the method. 

Here are the examples:
```
[29] pry(main)> ActiveSupport::JSON::Encoding.use_standard_json_time_format = true
=> true
[30] pry(main)> Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").to_json
=> "\"2005-02-01T05:15:10.000-10:00\""
[31] pry(main)> Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").as_json
=> "2005-02-01T05:15:10.000-10:00"
[32] pry(main)> ActiveSupport::JSON::Encoding.use_standard_json_time_format = false
=> false
[33] pry(main)> Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").as_json
=> "2005/02/01 05:15:10 -1000"
[34] pry(main)> Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").to_json
=> "\"2005/02/01 05:15:10 -1000\""
```

Author: dijonkitchen

activesupport/lib/active_support/time_with_zone.rb

@@ -157,11 +157,11 @@ def xmlschema(fraction_digits = 0)
     # to +false+.
     #
     #   # With ActiveSupport::JSON::Encoding.use_standard_json_time_format = true
-    #   Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").to_json
+    #   Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").as_json
     #   # => "2005-02-01T05:15:10.000-10:00"
     #
     #   # With ActiveSupport::JSON::Encoding.use_standard_json_time_format = false
-    #   Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").to_json
+    #   Time.utc(2005,2,1,15,15,10).in_time_zone("Hawaii").as_json
     #   # => "2005/02/01 05:15:10 -1000"
     def as_json(options = nil)
       if ActiveSupport::JSON::Encoding.use_standard_json_time_format