Fix typo about Executor (#1386)

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1386

Reviewed By: yfeldblum

Differential Revision: D21992975

Pulled By: Orvid

fbshipit-source-id: 7d52e3e9f8db899079263ef15e9f1bbfbedadc44

folly/Executor.h

@@ -80,7 +80,7 @@ class Executor {
    * For any Executor that supports KeepAlive functionality, Executor's
    * destructor will block until all the KeepAlive objects associated with that
    * Executor are destroyed.
-   * For Executors that don't support the KeepAlive funcionality, KeepAlive
+   * For Executors that don't support the KeepAlive functionality, KeepAlive
    * doesn't provide such protection.
    *
    * KeepAlive should *always* be used instead of Executor*. KeepAlive can be