fix(VSnackbar): fix class name for center location

[SonTT19]

Description

fixes #19870

Markup:

<template>
  <div class="text-center ma-2">
    <v-btn @click="snackbar = true"> Open Snackbar </v-btn>
    <v-snackbar v-model="snackbar" location="bottom center">
      {{ text }}

      <template #actions>
        <v-btn color="pink" variant="text" @click="snackbar = false">
          Close
        </v-btn>
      </template>
    </v-snackbar>
  </div>
</template>

<script>
  export default {
    data: () => ({
      snackbar: false,
      text: `Hello, I'm a snackbar`,
    }),
  }
</script>