fix ELB ConnectionSettings values in documentation

While elb.attributes.ConnectionSettingAttribute is the name of the class, the get_ and modify_ classes sniff for "connectingsettings" (note 'ing' and plural 's' at end). The safest thing to do is to make the documentation correct. Another possibility would be to add an 'or' to the conditionals.
Merge pull request #2620 from tedder/patch-1

fix ELB ConnectionSettings values in documentation