Update facade docblocks

Author: taylorotwell

src/Illuminate/Support/Facades/Http.php

@@ -33,6 +33,7 @@
  * @method static \Illuminate\Http\Client\PendingRequest acceptJson()
  * @method static \Illuminate\Http\Client\PendingRequest accept(string $contentType)
  * @method static \Illuminate\Http\Client\PendingRequest withHeaders(array $headers)
+ * @method static \Illuminate\Http\Client\PendingRequest withHeader(string $name, mixed $value)
  * @method static \Illuminate\Http\Client\PendingRequest replaceHeaders(array $headers)
  * @method static \Illuminate\Http\Client\PendingRequest withBasicAuth(string $username, string $password)
  * @method static \Illuminate\Http\Client\PendingRequest withDigestAuth(string $username, string $password)