fault-injection: update documentation with the mmc module param

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/fault-injection/fault-injection.txt

@@ -121,7 +121,7 @@ use the boot option:
 	failslab=
 	fail_page_alloc=
 	fail_make_request=
-	fail_mmc_request=<interval>,<probability>,<space>,<times>
+	mmc_core.fail_request=<interval>,<probability>,<space>,<times>
 
 How to add new fault injection capability
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