Fixed style error

Using unless with else where the style guide states that we should not.
Merge pull request #17 from NikolayGenov/03-fix-style

Fixed style error
Separate heading and subheading on slide 30
Typo
Fix examples in lecture 03
Merge pull request #18 from dkeranov/03-fix-examples

Fix examples in lecture 03
Correct spelling mistakes
Fix lambda call according to style guide
Correct method name
Merge pull request #21 from AlexVPopov/03-typos-refactoring

03 typos refactoring
Merge pull request #22 from AngelVenchev/patch-1

Fix style and spelling mistakes in lecture 3
Fix small inaccuracies
Merge pull request #24 from GeorgeSG/minor-fixes

Fix small inaccuracies
Fix 4 typos

I checked the slides and found 4 different small typos.
Merge pull request #27 from yasen23/01-fix-typos

Fix 4 typos
Lecture 04