Clarify CategoryForm component

[redrambles]

<select id='category' name='category' value={category} onChange={handleCategoryChange}>
          {categories.map((category) => (
            <option key={category} value={category}>
              {category}
            </option>
          ))}
        </select>

Should be:

        <select id='category' name='category' value={category} onChange={handleCategoryChange}>
          {categories.map((category-item) => (
            <option key={category-item} value={category-item}>
              {category-item}
            </option>
          ))}
        </select>

To help differentiate from the category prop that is the state passed by App.js, and the element that we are iterating over from the categories array. This change will need to be made in branch 4 (filter) and onward.