Exclude string.rb from consideration by YARD.

This is necessary because otherwise YARD will assume all
references to "String" mean Faker::String.

The long-term solution for this is probably to deprecate Faker::String
and move the Faker::String.random method into Faker::Types.

.yardopts

@@ -1,5 +1,6 @@
 --no-private
 --tag faker.version:"Available since"
 --exclude lib/faker/deprecate/
+--exclude lib/faker/default/string.rb
 --output-dir yard_docs
-lib/**/*.rb
+lib/**/*.rb