Updated directive.md (#1204)

`$t` is function of "Vue18n" instance. It has the following pros and cons.
Incorrect : "Vue18n"
Change: "VueI18n"

Author: sayan99614

docs/guide/advanced/directive.md

@@ -116,7 +116,7 @@ Outputs:
 
 ### `$t`
 
-`$t` is function of Vue18n instance. It has the following pros and cons:
+`$t` is function of VueI18n instance. It has the following pros and cons:
 
 #### Pros