chore: track main branch for FixIt submodule

Author: attajak

.gitmodules

@@ -1,6 +1,7 @@
 [submodule "themes/FixIt"]
 	path = themes/FixIt
 	url = https://github.com/hugo-fixit/FixIt
+	branch = main
 [submodule "themes/shortcode-docs-bookmark"]
 	path = themes/shortcode-docs-bookmark
 	url = https://github.com/hugo-fixit/shortcode-docs-bookmark