Change the warning message.

src/shims/thread.rs

@@ -14,7 +14,7 @@ fn pthread_create(
         let this = self.eval_context_mut();
 
         this.tcx.sess.warn(
-            "The thread support is experimental. \
+            "thread support is experimental. \
              For example, Miri does not detect data races yet.",
         );