Fix purchase URL

Author: LMBishop

_collections/_merch/plushies.md

@@ -2,7 +2,7 @@
 layout: merch
 title: TeX Plushies
 slug: plushies
-url: https://www.guildofstudents.com/
+purchase-url: https://www.guildofstudents.com/
 options:
   - name: Plushie
     quantity: 1

_layouts/merch.html

@@ -48,8 +48,8 @@ <h2>Purchase options</h2>
             <span class="price">£{{ option.price }}</span>
           </div>
         {% endfor %}
-        {% if page.url %}
-          <a href="{{ page.url }}" class="button guild-button">Buy on Guild website <i class="fas fa-arrow-right"></i></a>
+        {% if page.purchase-url %}
+          <a href="{{ page.purchase-url }}" class="button guild-button">Buy on Guild website <i class="fas fa-arrow-right"></i></a>
         {% endif %}
       </div>
     </div>