ci: update package index to main branch

Author: fpistm

.github/workflows/compile-examples.yml

@@ -21,5 +21,5 @@ jobs:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           fqbn: ${{ matrix.fqbn }}
           platforms: |
-            - source-url: https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json
+            - source-url: https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json
               name: STMicroelectronics:stm32