docs: ✏️ tweak demo

Author: ttskch

docs/index.html

@@ -45,12 +45,12 @@ <h3>Example of <a href="https://github.com/ttskch/select2-bootstrap4-theme" targ
     </div>
     <div class="form-group">
       <label>Example of disabled select</label>
-      <select disabled data-placeholder="Cannot choose">
+      <select disabled data-placeholder="Cannot choose" data-allow-clear="1">
       </select>
     </div>
     <div class="form-group">
       <label>Example of select with optgroup</label>
-      <select data-placeholder="Choose one thing">
+      <select data-placeholder="Choose one thing" data-allow-clear="1">
         <option></option>
         <optgroup label="Group A">
           <option>A1</option>
@@ -70,15 +70,15 @@ <h3>Example of <a href="https://github.com/ttskch/select2-bootstrap4-theme" targ
         <div class="input-group-prepend">
           <span class="input-group-text">Prepend</span>
         </div>
-        <select data-placeholder="Choose on thing">
+        <select data-placeholder="Choose on thing" data-allow-clear="1">
           <option></option>
           <option>1</option>
           <option>2</option>
           <option>3</option>
         </select>
       </div>
       <div class="input-group mt-2">
-        <select data-placeholder="Choose on thing">
+        <select data-placeholder="Choose on thing" data-allow-clear="1">
           <option></option>
           <option>1</option>
           <option>2</option>
@@ -92,7 +92,7 @@ <h3>Example of <a href="https://github.com/ttskch/select2-bootstrap4-theme" targ
         <div class="input-group-prepend">
           <span class="input-group-text">Prepend</span>
         </div>
-        <select data-placeholder="Choose on thing">
+        <select data-placeholder="Choose on thing" data-allow-clear="1">
           <option></option>
           <option>1</option>
           <option>2</option>
@@ -105,7 +105,7 @@ <h3>Example of <a href="https://github.com/ttskch/select2-bootstrap4-theme" targ
     </div>
     <div class="form-group">
       <label>Example of validated select</label>
-      <select data-placeholder="Invalid example" class="form-control is-invalid">
+      <select data-placeholder="Invalid example" class="form-control is-invalid" data-allow-clear="1">
         <option></option>
       </select>
       <div class="invalid-feedback">