fix typo (vuejs#1530)

Author: miljan-aleksic

src/v2/guide/components-registration.md

@@ -154,7 +154,7 @@ Now both `ComponentA` and `ComponentC` can be used inside `ComponentB`'s templat
 
 ### Automatic Global Registration of Base Components
 
-Many of your components will be relatively generic, possibly only wrapping an an element like an input or a button. We sometimes refer to these as [base components](../style-guide/#Base-component-names-strongly-recommended) and they tend to be used very frequently across your components.
+Many of your components will be relatively generic, possibly only wrapping an element like an input or a button. We sometimes refer to these as [base components](../style-guide/#Base-component-names-strongly-recommended) and they tend to be used very frequently across your components.
 
 The result is that many components may include long lists of base components: