trivial: fix typo in CONFIG_DEBUG_FS in gcov doc

The correct name is CONFIG_DEBUG_FS, add the missing underscore.

Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/gcov.txt b/Documentation/gcov.txt
index 40ec633..e7ca647 100644
--- a/Documentation/gcov.txt
+++ b/Documentation/gcov.txt
@@ -47,7 +47,7 @@
 
 Configure the kernel with:
 
-        CONFIG_DEBUGFS=y
+        CONFIG_DEBUG_FS=y
         CONFIG_GCOV_KERNEL=y
 
 and to get coverage data for the entire kernel: