fbdev: fbmon: fix function name in kernel-doc

Fix a kernel-doc warning by correcting the function name in the
kernel-doc comment:

drivers/video/fbdev/core/fbmon.c:1073: warning: expecting prototype for fb_get_hblank_by_freq(). Prototype was for fb_get_hblank_by_hfreq() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Helge Deller <deller@gmx.de>