Fix a few typos

src/util.cpp

@@ -1142,7 +1142,7 @@ void ShrinkDebugFile()
 // "Never go to sea with two chronometers; take one or three."
 // Our three time sources are:
 //  - System clock
-//  - Median of other nodes's clocks
+//  - Median of other nodes clocks
 //  - The user (asking the user to fix the system clock if the first two disagree)
 //
 static int64 nMockTime = 0;  // For unit testing