gdb/doc: use @samp{} in Python docs

In this review:

  https://inbox.sourceware.org/gdb-patches/86sem6ase5.fsf@gnu.org

it was pointed out that I should use @samp{} around some text I was
adding to the documentation.  However, the offending snippet of
documentation was something I copied from elsewhere in python.texi.
This commit fixes the original to use @samp{}.
1 file changed