Fix typo "text/text" => "text/plain" in tests/resource.py

Pointed out by @merwork at
https://github.com/msabramo/slumber/commit/f640ec56b42b59979c4cc06df0d1f2ae6c09ebbc#commitcomment-2579532
Merge pull request #57 from msabramo/fix_text_text_typo

Fix typo "text/text" => "text/plain" in tests/resource.py