Clarify Error Message 

Change "You can't call .one(12)" -> "For example, you can't call .one(12)." This clarifies the expression to make sure people understand .one(12) call is an example of something you can't do.
Fix minor typo in README
Merge pull request #861 from PowerKiKi/patch-2

Fix minor typo in README
Merge pull request #856 from hjast/patch-1

Clarify Error Message