[ruby/logger] Prefer String#[0, 1] over [0..0]

[0..0] internally creates an extra Array object, and so is slower and much more memory consuming

ruby/logger@20616ad34a

lib/logger/formatter.rb

@@ -13,7 +13,7 @@ def initialize
     end
 
     def call(severity, time, progname, msg)
-      Format % [severity[0..0], format_datetime(time), Process.pid, severity, progname,
+      Format % [severity[0, 1], format_datetime(time), Process.pid, severity, progname,
         msg2str(msg)]
     end