Don't mutate a string literal in main code sample

I know this is still being debated, but as of the current
version of Ruby, the first code sample does emit a deprecation
warning (hidden by default but still).

I think it's would be better to not mutate a string literal.

Author: byroot

bg/examples/i_love_ruby.md

@@ -8,7 +8,7 @@ say = "I love Ruby"
 puts say
 
 # Output "I *LOVE* RUBY"
-say['love'] = "*love*"
+say = say.sub("love", "*love*")
 puts say.upcase
 
 # Output "I *love* Ruby"