bug#63846: Typo in sed manual, missing "@cindex"

[Sebastian Carlos]

There is an incongruous phrase on "3.3 The s Command":

> number
> Only replace the numberth match of the regexp.
> interaction in s command Note: the POSIX standard does not specify what
should happen when...

 "interaction in s command" is out of place and weirdly capitalized.

I think this is caused by a missing @cindex, based on the source:

> @item @var{number}
> @cindex Replacing only @var{n}th match of regexp in a line
> Only replace the @var{number}th match of the @var{regexp}.
>
> @cindex GNU extensions, @code{g} and @var{number} modifier
> interaction in @code{s} command
> @cindex Mixing @code{g} and @var{number} modifiers in the @code{s} command
> Note: the @sc{posix} standard does not specify what should happen
> when you mix the @code{g} and @var{number} modifiers,