Update json.rs

Hey, I'm trying to get a better understanding of `nom` reading this example, and think the following makes sense:
- delete lines `70` and `71`, as `value` was already described (lines `64 to 66`), and this other description seemed incomplete
- change `true` to `false` in line `75`, as it makes more sense with the code that follows

examples/json.rs

@@ -67,14 +67,12 @@ fn parse_str<'a, E: ParseError<&'a str>>(i: &'a str) -> IResult<&'a str, &'a str
 ///
 /// `alt` is another combinator that tries multiple parsers one by one, until
 /// one of them succeeds
-///
-/// `value` is a combinator that returns its value if the inner parser
 fn boolean<'a, E: ParseError<&'a str>>(input: &'a str) -> IResult<&'a str, bool, E> {
   // This is a parser that returns `true` if it sees the string "true", and
   // an error otherwise
   let parse_true = value(true, tag("true"));
 
-  // This is a parser that returns `true` if it sees the string "true", and
+  // This is a parser that returns `false` if it sees the string "false", and
   // an error otherwise
   let parse_false = value(false, tag("false"));