manual/configure.txt: fix C++ escaping

For asciidoc, a string like '+these are some words+' will be displayed in
monospace font. Such sequences are identified by searching for a matching
pair of + signs in the same block.

The string 'C++' also contains such + signs. In most cases, this does not
pose a problem and no escaping is necessary. However, if 'C++' occurs twice
in the same block, the + signs will be matched to each other, and asciidoc
formats all text between them as monospaced text. In this case, escaping of
one of these 'C++' occurances is necessary to get the right formatting.

In one place of the manual, there is a sentence that causes such a problem:
    "you only have to tell whether your toolchain supports C++ or not and
    whether it has built-in RPC support. If your external toolchain uses
    the 'uClibc' library, then you have to tell Buildroot if it supports
    largefile, IPv6, RPC, wide-char, locale, program invocation, threads
    and C++."

Commit 082dec8c was based on a patch fixing
this problem in one place of the manual, but was incorrectly changed while
committing.

This patch reverts the incorrect changes in that commit, and solves the
problem correctly.

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>