doc: correct argument type for process.cpuUsage

PR-URL: https://github.com/nodejs/node/pull/8158
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/process.md

@@ -586,7 +586,7 @@ over the IPC channel using `process.send()`.
 added: v6.1.0
 -->
 
-* `previousValue` {Array} A previous return value from calling
+* `previousValue` {Object} A previous return value from calling
   `process.cpuUsage()`
 
 The `process.cpuUsage()` method returns the user and system CPU time usage of