[Layout] AlphaStack update docs for align prop (Shopify#7963)

### WHY are these changes introduced?

The docs for `AlphaStack` `align` prop are confusing. `align` changes
the horizontal alignment, not vertical

### WHAT is this pull request doing?

Update the docs from vertical to horizontal

Author: aveline

.changeset/loud-onions-check.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris': minor
+'polaris.shopify.com': minor
+---
+
+Updated `AlphaStack` docs for `align` prop

polaris-react/src/components/AlphaStack/AlphaStack.tsx

@@ -22,7 +22,7 @@ export interface AlphaStackProps extends React.AriaAttributes {
    * @default 'div'
    */
   as?: Element;
-  /** Vertical alignment of children
+  /** Horizontal alignment of children
    * @default 'start'
    */
   align?: Align;

polaris.shopify.com/content/components/alpha-stack.md

@@ -23,7 +23,7 @@ examples:
   - fileName: alpha-stack-with-align.tsx
     title: Alignment
     description: >-
-      Use to vertically align Stack.
+      Use to horizontally align Stack.
 ---
 
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