docs: capitalize the first letter of mainnet

README.md

@@ -88,7 +88,7 @@ The response looks like:
 ```bash
 /!\ Welcome to the fee table. Please read the information below to understand the transaction fee
 
-The table below displays information about the cost of the transaction on the mainnet network, depending on the gas price selected. Multiple modes are displayed to help you choose a gas price depending on your needs:
+The table below displays information about the cost of the transaction on the Mainnet network, depending on the gas price selected. Multiple modes are displayed to help you choose a gas price depending on your needs:
 
 Information about the network:
 Costs based on block number: 4275264