Fix typo.

src/libstd/sys/unix/thread.rs

@@ -426,7 +426,7 @@ pub mod guard {
 }
 
 // glibc >= 2.15 has a __pthread_get_minstack() function that returns
-// PTHREAD_STACK_MIN plus however many bytes are needed for thread-local
+// PTHREAD_STACK_MIN plus how many bytes are needed for thread-local
 // storage.  We need that information to avoid blowing up when a small stack
 // is created in an application with big thread-local storage requirements.
 // See #6233 for rationale and details.