Fix branch name for syntax_suggest sync

Followup to ruby#16674, I dropped this by accident

Author: Earlopain

tool/sync_default_gems.rb

@@ -295,7 +295,7 @@ def lib((upstream, branch), gemspec_in_subdir: false)
       "ext/strscan/regint.h",
       "ext/strscan/lib/strscan/truffleruby.rb",
     ]),
-    syntax_suggest: repo("ruby/syntax_suggest", [
+    syntax_suggest: repo(["ruby/syntax_suggest", "main"], [
       ["lib/syntax_suggest.rb", "lib/syntax_suggest.rb"],
       ["lib/syntax_suggest", "lib/syntax_suggest"],
       ["syntax_suggest.gemspec", "lib/syntax_suggest/syntax_suggest.gemspec"],