./configure: cleanup '--enable-background' option description

Before the patch ./configure --help shown:
  --enable-background     Support for background file operations [[yes]]

After the patch:
  --enable-background     Support for background file operations [yes]

Changed whitespace: converted tabs to spaces.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

m4.include/mc-background.m4

@@ -5,7 +5,7 @@ dnl
 AC_DEFUN([MC_BACKGROUND],
 [
     AC_ARG_ENABLE([background],
-	AS_HELP_STRING([--enable-background], [Support for background file operations [[yes]]]),
+    AS_HELP_STRING([--enable-background], [Support for background file operations [yes]]),
     [
         if test "x$enableval" = xno; then
         enable_background=no