refactor: replace deprecated ioutil.ReadAll with io.ReadAll

io/ioutil has been deprecated since Go 1.16. Also updates the README
usage example to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: alecthomas

README.md

@@ -165,7 +165,7 @@ if formatter == nil {
 Then obtain an iterator over the tokens:
 
 ```go
-contents, err := ioutil.ReadAll(r)
+contents, err := io.ReadAll(r)
 iterator, err := lexer.Tokenise(nil, string(contents))
 ```
 

cmd/chroma/main.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"iter"
 	"os"
 	"os/signal"
@@ -89,7 +88,7 @@ func (n *nopFlushableWriter) Flush() error { return nil }
 
 // prepareLenient prepares contents and lexer for input, using fallback lexer if no specific one is available for it.
 func prepareLenient(r io.Reader, filename string) (string, chroma.Lexer, error) {
-	data, err := ioutil.ReadAll(r)
+	data, err := io.ReadAll(r)
 	if err != nil {
 		return "", nil, err
 	}