Correction in documentation

On this
<https://www.gnu.org/software/bash/manual/html_node/ANSI_002dC-Quoting.html>
page
of Bash documentation, it is mentioned that to type an ASCII character we
can write it in its octal notation. However, there is a mistake here.

The documentation says that the character must be written as `\nnn` where
"nnn" are octal digits. The actual working requires that we type as `\0nnn`
where "nnn" are octal digits. Without the `\0` "nnn" isn't recognised as an
octal representation.

Please fix this error.