threads1: Fix typos in description

Author: cenviity

exercises/20_threads/threads1.rs

@@ -1,5 +1,5 @@
-// This program spawns multiple threads that each run for at least 250ms, and
-// each thread returns how much time they took to complete. The program should
+// This program spawns multiple threads that each runs for at least 250ms, and
+// each thread returns how much time it took to complete. The program should
 // wait until all the spawned threads have finished and should collect their
 // return values into a vector.
 

solutions/20_threads/threads1.rs

@@ -1,5 +1,5 @@
-// This program spawns multiple threads that each run for at least 250ms, and
-// each thread returns how much time they took to complete. The program should
+// This program spawns multiple threads that each runs for at least 250ms, and
+// each thread returns how much time it took to complete. The program should
 // wait until all the spawned threads have finished and should collect their
 // return values into a vector.