doc: fix typo in cnxk platform guide

[ upstream commit 443000992bc937d4686a37e4eef9c66b6b3ce642 ]

The Linux kernel option has an added underscore.

Fixes: 14ad4f0 ("doc: add Marvell OCTEON TX2 platform guide")

Signed-off-by: Thierry Herbelot <thierry.herbelot@6wind.com>

doc/guides/platform/cnxk.rst

@@ -244,7 +244,7 @@ context or stats using debugfs.
 
 Enable ``debugfs`` by:
 
-1. Compile kernel with debugfs enabled, i.e ``CONFIG_DEBUGFS=y``.
+1. Compile kernel with debugfs enabled, i.e ``CONFIG_DEBUG_FS=y``.
 2. Boot OCTEON CN9K/CN10K with debugfs supported kernel.
 3. Verify ``debugfs`` mounted by default "mount | grep -i debugfs" or mount it manually by using.