Replace "they" with "the hooks" for better clarification (vuejs#1238)

Author: phanan

src/v2/guide/transitions.md

@@ -440,7 +440,7 @@ methods: {
 
 These hooks can be used in combination with CSS transitions/animations or on their own.
 
-<p class="tip">When using JavaScript-only transitions, **the `done` callbacks are required for the `enter` and `leave` hooks**. Otherwise, they will be called synchronously and the transition will finish immediately.</p>
+<p class="tip">When using JavaScript-only transitions, **the `done` callbacks are required for the `enter` and `leave` hooks**. Otherwise, the hooks will be called synchronously and the transition will finish immediately.</p>
 
 <p class="tip">It's also a good idea to explicitly add `v-bind:css="false"` for JavaScript-only transitions so that Vue can skip the CSS detection. This also prevents CSS rules from accidentally interfering with the transition.</p>