Let the comment in GetBlockValue() reflect the uncertainty about the time interval between subsidy reductions

src/main.cpp

@@ -690,7 +690,7 @@ int64 static GetBlockValue(int nHeight, int64 nFees)
 {
     int64 nSubsidy = 50 * COIN;
 
-    // Subsidy is cut in half every 4 years
+    // Subsidy is cut in half every 210000 blocks, which will occur approximately every 4 years
     nSubsidy >>= (nHeight / 210000);
 
     return nSubsidy + nFees;