Documentation: sysrq: update description about sysrq crash

After commit 8341f2f2 ("sysrq: Use panic() to force a crash"),
a crash was not generated by dereferencing a NULL pointer anymore.

Let's update documentation as well to make it less misleading.
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Zefan Li <lizefan.x@bytedance.com>
Link: https://lore.kernel.org/r/20210309191550.3955601-1-hsiangkao@redhat.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysrq.rst

@@ -90,8 +90,8 @@ Command	    Function
 ``b``	    Will immediately reboot the system without syncing or unmounting
             your disks.
 
-``c``	    Will perform a system crash by a NULL pointer dereference.
-            A crashdump will be taken if configured.
+``c``	    Will perform a system crash and a crashdump will be taken
+            if configured.
 
 ``d``	    Shows all locks that are held.