#60 Fix typo in JwtBuilder comments

A typographical error in the comments of the 'exp' function in JwtBuilder was corrected. The phrase "specified it if" was changed to "specified if it", making the comments clearer and easier to understand.

api/src/main/java/io/jsonwebtoken/JwtBuilder.java

@@ -589,7 +589,7 @@ public interface JwtBuilder extends ClaimsMutator<JwtBuilder> {
     /**
      * Sets the JWT Claims <a href="https://www.rfc-editor.org/rfc/rfc7519.html#section-4.1.4">
      * <code>exp</code></a> (expiration) claim. It will set the expiration Date to the issuedAt time plus the duration
-     * specified it if has been set, otherwise it will use the current system time plus the duration specified
+     * specified if it has been set, otherwise it will use the current system time plus the duration specified
      *
      * <p>A JWT obtained after this timestamp should not be used.</p>
      *