fix api doc bug

spikingjelly/activation_based/neuron.py

@@ -1933,7 +1933,7 @@ def __init__(self, k: int, T: int, lambda_init: float = 0.,
             H &= (W \\cdot {M}_{k})X, ~~~~~~~~~~~~~~~W \\in \\mathbb{R}^{T \\times T}, {M}_{k} \\in \\mathbb{R}^{T \\times T}, X \\in \\mathbb{R}^{T \\times N} \\\\
             S &= \\Theta(H - B), ~~~~~B \\in \\mathbb{R}^{T}, S\\in \\{0, 1\\}^{T \\times N}
 
-        where :math`W` is the learnable weight matrix, :math:`B` is the learnable threshold, and :math:`{M}_{k}` is defined as
+        where :math:`W` is the learnable weight matrix, :math:`B` is the learnable threshold, and :math:`{M}_{k}` is defined as
 
         .. math::