[9.x] Fix wrong doc blocks (laravel#580)

* Fix wrong doc block

Doc Block for the `getPrefixColumns` method was using the same description as the `getFullTextColumns` method.

I based the description on the attribute `SearchUsingPrefix`'s `$columns` property description.

https://github.com/laravel/scout/blob/bb6bea547d7f9231dab2bb378fed12a64a000687/src/Attributes/SearchUsingPrefix.php#L12

* Fix typo

src/Engines/DatabaseEngine.php

@@ -175,7 +175,7 @@ protected function initializeSearchQuery(Builder $builder, array $columns, array
     }
 
     /**
-     * Add additional, developer defined constraints to the serach query.
+     * Add additional, developer defined constraints to the search query.
      *
      * @param  \Laravel\Scout\Builder  $builder
      * @param  \Illuminate\Database\Eloquent\Builder  $query
@@ -231,7 +231,7 @@ protected function getFullTextColumns(Builder $builder)
     }
 
     /**
-     * Get the full-text columns for the query.
+     * Get the prefix search columns for the query.
      *
      * @param  \Laravel\Scout\Builder  $builder
      * @return array