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;