Updated installation command

MailboxValidator · web-flow · commit 707950da81bb · 2023-02-14T10:27:39.000+08:00
diff --git a/README.md b/README.md
@@ -18,7 +18,7 @@ Installation
 
 To install this module type the following:
 
-   `gem install mailboxvalidator-ruby`
+   `gem install mailboxvalidator_ruby`
 
 Dependencies
 ============
