[ruby/syntax_suggest] Fix warning message

The env var DEBUG does not work to produce detailed output. It is SYNTAX_SUGGEST_DEBUG. It was changed as part of the dead_end to syntax_suggest migration for Ruby 3.2

https://github.com/ruby/syntax_suggest/commit/c41da7aab7

lib/syntax_suggest/api.rb

@@ -78,7 +78,7 @@ module SyntaxSuggest
       code_lines: search.code_lines
     ).call
   rescue Timeout::Error => e
-    io.puts "Search timed out SYNTAX_SUGGEST_TIMEOUT=#{timeout}, run with DEBUG=1 for more info"
+    io.puts "Search timed out SYNTAX_SUGGEST_TIMEOUT=#{timeout}, run with SYNTAX_SUGGEST_DEBUG=1 for more info"
     io.puts e.backtrace.first(3).join($/)
   end