Convert links master -> main

CONTRIBUTING.md

@@ -21,4 +21,4 @@ Guidelines
 -------
 
 * Ensure you've understood the specification and built the code according to the challenge guidelines.
-* Read through [Code Reviews :pill:](https://github.com/makersacademy/course/blob/master/pills/code_reviews.md) to understand what we're looking for in your code.
+* Read through [Code Reviews :pill:](https://github.com/makersacademy/course/blob/main/pills/code_reviews.md) to understand what we're looking for in your code.

README.md

@@ -99,7 +99,7 @@ Code Review
 In code review we'll be hoping to see:
 
 * All tests passing
-* High [Test coverage](https://github.com/makersacademy/course/blob/master/pills/test_coverage.md) (>95% is good)
+* High [Test coverage](https://github.com/makersacademy/course/blob/main/pills/test_coverage.md) (>95% is good)
 * The code is elegant: every class has a clear responsibility, methods are short etc.
 
 Reviewers will potentially be using this [code review rubric](docs/review.md).  Referring to this rubric in advance may make the challenge somewhat easier.  You should be the judge of how much challenge you want at this moment.

docs/review.md

@@ -254,7 +254,7 @@ module Helpers
   end
 ```
 
-See https://github.com/makersacademy/course/blob/master/pills/spec_helper_methods.md
+See https://github.com/makersacademy/course/blob/main/pills/spec_helper_methods.md
 
 # Step 4: Application code and \*.rb files
 
@@ -525,7 +525,7 @@ end
 
 ## Nice-to-have: Ensure Rakefile has Appropriate Tasks
 
-(We removed Rake from the Bookmark Manager sequence in 2018, but it's nice to know about. Here is [the step](https://github.com/makersacademy/course/blob/master/bookmark_manager/using_rake.md) and here's [the walkthrough](https://github.com/makersacademy/course/blob/master/bookmark_manager/walkthroughs/using_rake.md).)
+(We removed Rake from the Bookmark Manager sequence in 2018, but it's nice to know about. Here is [the step](https://github.com/makersacademy/course/blob/main/bookmark_manager/using_rake.md) and here's [the walkthrough](https://github.com/makersacademy/course/blob/main/bookmark_manager/walkthroughs/using_rake.md).)
 
 Any scripts that touch the database or working area should be moved to the Rakefile.