update mcrouter cli doc

Summary: More accurate to the actual business logic: https://www.internalfb.com/code/fbsource/fbcode/mcrouter/StandaloneUtils.cpp?lines=463-469

Differential Revision: D76058028

fbshipit-source-id: b257dd0bd28ad528b766aa7bc8a69e39ac92b865

Author: kcdode

third-party/mcrouter/src/mcrouter/standalone_options_list.h

@@ -299,7 +299,7 @@ MCROUTER_OPTION_INTEGER(
     0,
     "core-multiplier-threshold",
     no_short,
-    "core-mutiplier logic is applied if the number of cores  >= core_multiplier_threshold")
+    "core-mutiplier logic is applied if std::hardware_concurrency >= core_multiplier_threshold.")
 
 // ONLY ENABLED FOR SHADOW TESTING WHEN USING PROXY
 MCROUTER_OPTION_TOGGLE(