Update .spi config to omit internal symbols in docs (#104)

Updates the Swift Package Index config file (.spi file) to only allow
public symbols to appear in documentation. This fixes an issue where
internal symbols were shown on the documentation pages for SVD2Swift.

.spi.yml

@@ -6,3 +6,5 @@ builder:
       - SVD
       - SVD2LLDB
       - SVD2Swift
+      custom_documentation_parameters:
+      - "--minimum-access-level public"