Change capitalization, remove whitespace of rpc keys

All other keys of this rpc command are written in lower case and the
space is replaced by an underscore. The rpc command is not really useful
for normal users and a consistent naming of the keys is better in my
opinion. Therefore I have adjusted these two keys.

src/rpc/blockchain.cpp

@@ -2957,11 +2957,11 @@ UniValue SuperblockReport(int lookback, bool displaycontract, std::string cpid)
 
                 if (cpid_parsed)
                 {
-                    c.pushKV("Magnitude", superblock.m_cpids.MagnitudeOf(*cpid_parsed).Floating());
+                    c.pushKV("magnitude", superblock.m_cpids.MagnitudeOf(*cpid_parsed).Floating());
                 }
 
                 if (displaycontract)
-                    c.pushKV("Contract Contents", SuperblockToJson(superblock));
+                    c.pushKV("contract_contents", SuperblockToJson(superblock));
 
                 results.push_back(c);