Fix gdb backtrace command

Correct the command to get a gdb backtrace from all threads. `apply` is
not a valid gdb command.

See https://sourceware.org/gdb/onlinedocs/gdb/Threads.html#Threads .

doc/administration/troubleshooting/debug.md

@@ -107,7 +107,7 @@ downtime. Otherwise skip to the next section.
 1. To see the current threads, run:
 
     ```
-    apply all thread bt
+    thread apply all bt
     ```
 
 1. Once you're done debugging with `gdb`, be sure to detach from the process and exit: