fix to documentation of `grep-find-use-xargs'

[Matt Swift]

In GNU Emacs 21.2.1 (i386-debian-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2002-03-22 on raven, modified by Debian
configured using `configure  i386-debian-linux-gnu --prefix=/usr 
--sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib 
--infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes --with-x=yes 
--with-x-toolkit=athena --without-gif'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

The file compile.el documents the variable `grep-find-use-xargs' in
the following way:

    (defvar grep-find-use-xargs nil
      "Whether \\[grep-find] uses the `xargs' utility by default.

    If nil, it uses `grep -exec'; if `gnu', it uses `find -print0' and `xargs 
-0';
    if not nil and not `gnu', it uses `find -print' and `xargs'.

    This variable's value takes effect when `grep-compute-defaults' is called.")

The text should read `find -exec' instead of `grep -exec'.