Fix deprecation warning (#3067)

### Changes

Use raw string for docstring with `\`

### Reason for changes

```
nncf/quantization/algorithms/weight_compression/backend.py:165
  /home/runner/work/nncf/nncf/nncf/quantization/algorithms/weight_compression/backend.py:165: DeprecationWarning: invalid escape sequence '\ '
    """
```

nncf/quantization/algorithms/weight_compression/backend.py

@@ -162,7 +162,7 @@ def transform_model(
     def insert_adapters(
         self, wc_params: WeightCompressionParameters, lora_A: Tensor, lora_B: Tensor, int8_lora: bool
     ) -> None:
-        """
+        r"""
         Expands a model's execution graph following the Low-Rank Adaptation (LoRA) concept.
 
         It inserts two additional Linear layers with weight matrices of low rank that are executed in parallel to the