gdb/doc: fix incorrect use of @value

gdb/doc: fix incorrect use of @value

In commit:

  commit 255a9c42709958d0925d1d0d1d39d262972fd2a6
  Date:   Fri Dec 5 11:33:29 2025 +0000

      gdb: new setting to disable progress bars

I incorrectly used @value{on} and @value{off} to reference the on/off
settings of a new flag.  This caused the following warnings when
building the docs:

  gdb.texinfo:51356: warning: undefined flag: off
  gdb.texinfo:51357: warning: undefined flag: on

Looking through the docs there seems to be a split, in some cases we
use @code and in others we use @samp.  I'm not sure @code is correct,
so I've switched to use @samp.  The warnings are gone after this
patch.