Fix a typo in Percent Literals

Found it while reading the section.
Merge pull request #282 from gsamokovarov/patch-1

Fix a typo in Percent Literals
Mention string formatting as an alternative to string interpolation
Don't use Object#to_s on interpolated objects
[Fix #237] Add guideline for unused variables naming
Extend ||= rule
Recommend the use of Hash#values_at
Recommend the use of self-assignment operators
Add a link to the Japanese version.
Merge pull request #285 from 5704k3/addlinkja

Add a link to the Japanese version.
Clarify a statement
Add a rule about mixing Ruby 1.9 hash syntax with hash rockets syntax
Add a rule about a newline at eof
Merge pull request #287 from SaM-Solutions/newline-at-eof

Add a rule about a newline at eof
Leverage the fact that if and case are expressions
Extend a bit the &&= rule
Don't use count as a substitute for size
[#184 #222] Acknowledge double-quoted strings as good style
[#169 #176] Acknowledge trailing . as a good style as well
Link leading vs trailing dot discussion