docs: zh_CN: amu.rst: fix document title markup

As warned by Sphinx 2.4.4:
	.../Documentation/translations/zh_CN/arm64/amu.rst:7: WARNING: Title overline too short.

Increase its size.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/translations/zh_CN/arm64/amu.rst

@@ -4,9 +4,9 @@
 
 Translator: Bailu Lin <bailu.lin@vivo.com>
 
-=================================
+==================================
 AArch64 Linux 中扩展的活动监控单元
-=================================
+==================================
 
 作者: Ionela Voinescu <ionela.voinescu@arm.com>