Add "Waiting For Goroutines To Finish" example.

Also link to it from the Channel Synchronization example

examples/channel-synchronization/channel-synchronization.go

@@ -1,6 +1,8 @@
 // We can use channels to synchronize execution
 // across goroutines. Here's an example of using a
 // blocking receive to wait for a goroutine to finish.
+// When waiting for multiple goroutines to finish,
+// you may prefer to [use a WaitGroup](waiting-for-goroutines-to-finish).
 
 package main
 

examples/channel-synchronization/channel-synchronization.hash

@@ -1,2 +1,2 @@
-fe3e2ea1a67d0f95ce4cb18f3e8aa16d416de0ce
-0DfW-1RMqi
+df90432a53832c045472981a9da23d1139fb2b5c
+f6qTtt1agmN