docs: disambiguate variable name from attribute in x-bind example (#4742)

Rename `placeholder` to `placeholderText` in the x-bind docs example
so the variable name doesn't collide with the HTML placeholder attribute.

Cherry-picked from #4430 by @sa- with camelCase fix applied.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: calebporzio

packages/docs/src/en/directives/bind.md

@@ -10,8 +10,8 @@ title: bind
 For example, here's a component where we will use `x-bind` to set the placeholder value of an input.
 
 ```alpine
-<div x-data="{ placeholder: 'Type here...' }">
-    <input type="text" x-bind:placeholder="placeholder">
+<div x-data="{ placeholderText: 'Type here...' }">
+    <input type="text" x-bind:placeholder="placeholderText">
 </div>
 ```
 
@@ -21,7 +21,7 @@ For example, here's a component where we will use `x-bind` to set the placeholde
 If `x-bind:` is too verbose for your liking, you can use the shorthand: `:`. For example, here is the same input element as above, but refactored to use the shorthand syntax.
 
 ```alpine
-<input type="text" :placeholder="placeholder">
+<input type="text" :placeholder="placeholderText">
 ```
 
 > Despite not being included in the above snippet, `x-bind` cannot be used if no parent element has `x-data` defined. [→ Read more about `x-data`](/directives/data)