linter: Correct unified-diff format spelling
linter: Correct unified-diff format spelling

It's unified-diff with a dash, not an underscore.

Closes https://github.com/coala/coala/issues/5418

coala-analyzer/coala,SanketDG/coala,jayvdb/coala,coala/coala,SanketDG/coala,jayvdb/coala,coala/coala,SanketDG/coala,coala-analyzer/coala,coala/coala,jayvdb/coala,coala-analyzer/coala