Fix two typos in main.h

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

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);