unify case for the sender object in examples (in POD)

[whosgonna]

Building on issue #17 . The changes for the merged patch there are only in the README.md file, meaning that the examples will utilize the consistent case in the github page, but will still have $Sender instead of $sender when viewed on metacpan (not sure about anyone else, but generally speaking I read module documentation there).

I'll submit a PR for updates to POD, just opening this issue as a tracking item.

[whosgonna]

As few side notes:

There's a few other variables that are upper case in the module itself, but I didn't want to do anything with these, because:

• I don't see it as my place to enforce my style preferences in someone else's code. Plus my own code is far from perfect, so those who live in glass houses....
• Because these were very minor edits, and changing POD should not break code, I just used GitHub's online editor for these changes. I wouldn't want to actually change code in their editor, without testing, etc.

@richlv, I'm not sure if you're aware of POD, but it's perl's standard documentation format that allows for documentation to be included in the code itself. This is what's displayed on metacpan.org. It's also what's rendered if you run perldoc Zabbix::Sender.