Fix documentation for gdb.blocked_signals

gdb exports a context manager named gdb.blocked_signals, but the
documentation erroneously refers to it as gdb.block_signals.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32891
Approved-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom de Vries <tdevries@suse.de>