Fixes sign in DigitalFilter documentation (#3313)

# Description

Fixes #3293

Corrects the documentation for implementing a low-pass filter with
DigitalFilter.
The coefficient A should be [-α] not [α] because DigitalFilter subtracts
the
recursive term (Y*A) in its implementation.

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## Type of change

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- Bug fix (non-breaking change which fixes an issue)
- This change requires a documentation update

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## Checklist

- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x ] I have made corresponding changes to the documentation
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Co-authored-by: James Tigue <[email protected]>

Author: vi7n

source/isaaclab/isaaclab/utils/modifiers/modifier.py

@@ -123,11 +123,13 @@ class DigitalFilter(ModifierBase):
     where :math:`\alpha` is a smoothing parameter between 0 and 1. Typically, the value of :math:`\alpha` is
     chosen based on the desired cut-off frequency of the filter.
 
-    This filter can be implemented as a digital filter with the coefficients :math:`A = [\alpha]` and
+    This filter can be implemented as a digital filter with the coefficients :math:`A = [-\alpha]` and
     :math:`B = [1 - \alpha]`.
     """
 
-    def __init__(self, cfg: modifier_cfg.DigitalFilterCfg, data_dim: tuple[int, ...], device: str) -> None:
+    def __init__(
+        self, cfg: modifier_cfg.DigitalFilterCfg, data_dim: tuple[int, ...], device: str
+    ) -> None:
         """Initializes digital filter.
 
         Args:
@@ -141,7 +143,9 @@ def __init__(self, cfg: modifier_cfg.DigitalFilterCfg, data_dim: tuple[int, ...]
         """
         # check that filter coefficients are not None
         if cfg.A is None or cfg.B is None:
-            raise ValueError("Digital filter coefficients A and B must not be None. Please provide valid coefficients.")
+            raise ValueError(
+                "Digital filter coefficients A and B must not be None. Please provide valid coefficients."
+            )
 
         # initialize parent class
         super().__init__(cfg, data_dim, device)
@@ -213,7 +217,9 @@ class Integrator(ModifierBase):
     :math:`\Delta t` is the time step between samples.
     """
 
-    def __init__(self, cfg: modifier_cfg.IntegratorCfg, data_dim: tuple[int, ...], device: str):
+    def __init__(
+        self, cfg: modifier_cfg.IntegratorCfg, data_dim: tuple[int, ...], device: str
+    ):
         """Initializes the integrator configuration and state.
 
         Args: