Change our GitHub Actions to trigger on main instead of master (#324)

Author: spier

.github/workflows/book.yml

@@ -3,7 +3,7 @@ name: Book ToC Generation
 on:
   push:
     branches:
-      - "master"
+      - "main"
       - "book-staging"
 
 jobs:

.github/workflows/link-checker.yml

@@ -6,7 +6,7 @@ name: Link Check on Patterns and README
 on:
   push:
     branches:
-      - "master"
+      - "main"
   pull_request:
 
 jobs:

.github/workflows/lint-patterns.yml

@@ -4,7 +4,7 @@ name: Pattern Syntax Validation
 on:
   push:
     branches:
-      - "master"
+      - "main"
   pull_request:
     paths:
       - ".github/workflows/lint-patterns.yml"