fix(docs): resolve the {{MAIN_BRANCH}} template token in the contributing guide

The guide is instantiated from the estate template, whose own copy legitimately holds
`{{MAIN_BRANCH}}`. Here it was never substituted, so the reader is told to check whether
a bug "is already fixed in `{{MAIN_BRANCH}}`". Resolved to this repository's default
branch. One token, no other change.

Author: hyperpolymath

.github/CONTRIBUTING.md

@@ -58,7 +58,7 @@ patch-bridge/
 
 **Before reporting**:
 1. Search existing issues
-2. Check if it's already fixed in `{{MAIN_BRANCH}}`
+2. Check if it's already fixed in `main`
 3. Determine which perimeter the bug affects
 
 **When reporting**: