Update feature extractor docs (huggingface#18324)

As pointed out by @NielsRogge, a feature extractor is used to prepare inputs for a model with a single modality rather than multimodal models.

docs/source/en/main_classes/feature_extractor.mdx

@@ -12,7 +12,7 @@ specific language governing permissions and limitations under the License.
 
 # Feature Extractor
 
-A feature extractor is in charge of preparing input features for a multi-modal model. This includes feature extraction
+A feature extractor is in charge of preparing input features for audio or vision models. This includes feature extraction
 from sequences, *e.g.*, pre-processing audio files to Log-Mel Spectrogram features, feature extraction from images
 *e.g.* cropping image image files, but also padding, normalization, and conversion to Numpy, PyTorch, and TensorFlow
 tensors.