Fix two typos in main.h

Break one long comment down into 3 lines so it's readable.

src/bignum.h

@@ -131,7 +131,9 @@ public:
 
         if (sn < (int64)0)
         {
-            // Since the minimum signed integer cannot be represented as positive so long as its type is signed, and it's not well-defined what happens if you make it unsigned before negating it, we instead increment the negative integer by 1, convert it, then increment the (now positive) unsigned integer by 1 to compensate
+            // Since the minimum signed integer cannot be represented as positive so long as its type is signed, 
+            // and it's not well-defined what happens if you make it unsigned before negating it,
+            // we instead increment the negative integer by 1, convert it, then increment the (now positive) unsigned integer by 1 to compensate
             n = -(sn + 1);
             ++n;
             fNegative = true;

src/main.h

@@ -152,7 +152,7 @@ bool CheckProofOfWork(uint256 hash, unsigned int nBits);
 unsigned int ComputeMinWork(unsigned int nBase, int64 nTime);
 /** Get the number of active peers */
 int GetNumBlocksOfPeers();
-/** Check whether we are doin an inital block download (synchronizing from disk or network) */
+/** Check whether we are doing an initial block download (synchronizing from disk or network) */
 bool IsInitialBlockDownload();
 /** Format a string that describes several potential problems detected by the core */
 std::string GetWarnings(std::string strFor);