Update 2-language-rules.md

Update spelling.
Replaced reference to NP-Complete

A problem isn't NP-Complete by virtue of being difficult although many NP-Complete problems are difficult.
Fix a missing bracket
Update 2-language-rules.md
Merge pull request #15 from dvorobiov/patch-1

Just small fix for namespace for NonFatal object.
Merge pull request #10 from vendethiel/patch-1

Fix a missing bracket
Merge pull request #9 from arnfred/patch-1

Replaced reference to NP-Complete in Section 1.2
Merge pull request #1 from alexandru/master

updating fork