fix: typo & triggers in documentation workflow (zdharma-continuum#308)

- correct typo (e.g., branchs -> branches)
- add `main` branch to pull request trigger

Author: vladdoster

.github/workflows/documentation.yaml

@@ -2,10 +2,12 @@ name: Documentation
 
 on:
   pull_request:
+    branches:
+      - main
     paths:
       - 'zinit*.zsh'
   push:
-    branchs:
+    branches:
       - main
   workflow_dispatch: