core: fix a small typo in blockchain.go (#3611)

core/blockchain.go

@@ -853,7 +853,7 @@ func (self *BlockChain) WriteBlock(block *types.Block) (status WriteStatus, err
 	return
 }
 
-// InsertChain will attempt to insert the given chain in to the canonical chain or, otherwise, create a fork. It an error is returned
+// InsertChain will attempt to insert the given chain in to the canonical chain or, otherwise, create a fork. If an error is returned
 // it will return the index number of the failing block as well an error describing what went wrong (for possible errors see core/errors.go).
 func (self *BlockChain) InsertChain(chain types.Blocks) (int, error) {
 	// Do a sanity check that the provided chain is actually ordered and linked