Add `type="button"` to `<button>`s in accordion example (#25643)

For consistency. Closes https://github.com/twbs/bootstrap/issues/25642

docs/4.0/components/collapse.md

@@ -70,7 +70,7 @@ Using the [card]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/card
   <div class="card">
     <div class="card-header" id="headingOne">
       <h5 class="mb-0">
-        <button class="btn btn-link" data-toggle="collapse" data-target="#collapseOne" aria-expanded="true" aria-controls="collapseOne">
+        <button class="btn btn-link" type="button" data-toggle="collapse" data-target="#collapseOne" aria-expanded="true" aria-controls="collapseOne">
           Collapsible Group Item #1
         </button>
       </h5>
@@ -85,7 +85,7 @@ Using the [card]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/card
   <div class="card">
     <div class="card-header" id="headingTwo">
       <h5 class="mb-0">
-        <button class="btn btn-link collapsed" data-toggle="collapse" data-target="#collapseTwo" aria-expanded="false" aria-controls="collapseTwo">
+        <button class="btn btn-link collapsed" type="button" data-toggle="collapse" data-target="#collapseTwo" aria-expanded="false" aria-controls="collapseTwo">
           Collapsible Group Item #2
         </button>
       </h5>
@@ -99,7 +99,7 @@ Using the [card]({{ site.baseurl }}/docs/{{ site.docs_version }}/components/card
   <div class="card">
     <div class="card-header" id="headingThree">
       <h5 class="mb-0">
-        <button class="btn btn-link collapsed" data-toggle="collapse" data-target="#collapseThree" aria-expanded="false" aria-controls="collapseThree">
+        <button class="btn btn-link collapsed" type="button" data-toggle="collapse" data-target="#collapseThree" aria-expanded="false" aria-controls="collapseThree">
           Collapsible Group Item #3
         </button>
       </h5>