Fix typo in comment (#2660)

contracts/token/ERC20/ERC20.sol

@@ -43,7 +43,7 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
     /**
      * @dev Sets the values for {name} and {symbol}.
      *
-     * The defaut value of {decimals} is 18. To select a different value for
+     * The default value of {decimals} is 18. To select a different value for
      * {decimals} you should overload it.
      *
      * All two of these values are immutable: they can only be set once during