fix: remove references to '2.6.0+' (#586)

Author: skirtles-code

src/api/directives.md

@@ -197,7 +197,7 @@
   <!-- shorthand -->
   <button @click="doThis"></button>
 
-  <!-- shorthand dynamic event (2.6.0+) -->
+  <!-- shorthand dynamic event -->
   <button @[event]="doThis"></button>
 
   <!-- stop propagation -->

src/guide/template-syntax.md

@@ -169,7 +169,7 @@ The `v-` prefix serves as a visual cue for identifying Vue-specific attributes i
 <!-- shorthand -->
 <a @click="doSomething"> ... </a>
 
-<!-- shorthand with dynamic argument (2.6.0+) -->
+<!-- shorthand with dynamic argument -->
 <a @[event]="doSomething"> ... </a>
 ```