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>

Documentation/gcov.txt

@@ -47,7 +47,7 @@ Possible uses:
 
 Configure the kernel with:
 
-        CONFIG_DEBUGFS=y
+        CONFIG_DEBUG_FS=y
         CONFIG_GCOV_KERNEL=y
 
 and to get coverage data for the entire kernel: