Fix typos in ERC20 docs (#2364)

contracts/token/ERC20/ERC20.sol

@@ -140,9 +140,10 @@ contract ERC20 is Context, IERC20 {
      * @dev See {IERC20-transferFrom}.
      *
      * Emits an {Approval} event indicating the updated allowance. This is not
-     * required by the EIP. See the note at the beginning of {ERC20};
+     * required by the EIP. See the note at the beginning of {ERC20}.
      *
      * Requirements:
+     *
      * - `sender` and `recipient` cannot be the zero address.
      * - `sender` must have a balance of at least `amount`.
      * - the caller must have allowance for ``sender``'s tokens of at least
@@ -220,7 +221,7 @@ contract ERC20 is Context, IERC20 {
      *
      * Emits a {Transfer} event with `from` set to the zero address.
      *
-     * Requirements
+     * Requirements:
      *
      * - `to` cannot be the zero address.
      */
@@ -240,7 +241,7 @@ contract ERC20 is Context, IERC20 {
      *
      * Emits a {Transfer} event with `to` set to the zero address.
      *
-     * Requirements
+     * Requirements:
      *
      * - `account` cannot be the zero address.
      * - `account` must have at least `amount` tokens.