fix(docs): fixed typo in README.md (#2371)

Co-authored-by: Musa <goat_musa@Musas-MBP.cust.communityfibre.co.uk>

Author: devmanbud

README.md

@@ -99,7 +99,7 @@ Validator                               | Description
 **isBase64(str [, options])**          | check if the string is base64 encoded. `options` is optional and defaults to `{ urlSafe: false }`<br/> when `urlSafe` is true it tests the given base64 encoded string is [url safe][Base64 URL Safe].
 **isBefore(str [, date])**              | check if the string is a date that is before the specified date.
 **isBIC(str)**                          | check if the string is a BIC (Bank Identification Code) or SWIFT code.
-**isBoolean(str [, options])**          | check if the string is a boolean.<br/>`options` is an object which defaults to `{ loose: false }`. If `loose` is is set to false, the validator will strictly match ['true', 'false', '0', '1']. If `loose` is set to true, the validator will also match 'yes', 'no', and will match a valid boolean string of any case. (e.g.: ['true', 'True', 'TRUE']).
+**isBoolean(str [, options])**          | check if the string is a boolean.<br/>`options` is an object which defaults to `{ loose: false }`. If `loose` is set to false, the validator will strictly match ['true', 'false', '0', '1']. If `loose` is set to true, the validator will also match 'yes', 'no', and will match a valid boolean string of any case. (e.g.: ['true', 'True', 'TRUE']).
 **isBtcAddress(str)**            | check if the string is a valid BTC address.
 **isByteLength(str [, options])**          | check if the string's length (in UTF-8 bytes) falls in a range.<br/><br/>`options` is an object which defaults to `{ min: 0, max: undefined }`.
 **isCreditCard(str [, options])**                   | check if the string is a credit card number.<br/><br/> `options` is an optional object that can be supplied with the following key(s): `provider` is an optional key whose value should be a string, and defines the company issuing the credit card. Valid values include `['amex', 'dinersclub', 'discover', 'jcb', 'mastercard', 'unionpay', 'visa']` or blank will check for any provider.