fix: CCallout: fix css classes

Author: woothu

src/components/callout/CCallout.vue

@@ -1,5 +1,5 @@
-<template functional>
-  <div :class="`callout ${props.color ? 'callout-' + props.color : ''}`">
+<template>
+  <div :class="`c-callout ${color ? 'c-callout-' + color : ''}`">
     <slot></slot>
   </div>
 </template>

src/components/callout/tests/__snapshots__/CCallout.spec.js.snap

@@ -2,13 +2,13 @@
 
 exports[`CCallout renders correctly 1`] = `
 <div
-  class="callout "
+  class="c-callout "
 />
 `;
 
 exports[`CCallout renders correctly 2`] = `
 <div
-  class="callout callout-success"
+  class="c-callout c-callout-success"
 >
   Callout
 </div>