!233 Fix bug of formula of coverage metrics

From: @zhidanliu
Reviewed-by: @pkuliuliu,@jxlang910
Signed-off-by: @pkuliuliu,@jxlang910

mindarmour/fuzz_testing/model_coverage_metrics.py

@@ -174,8 +174,8 @@ class ModelCoverageMetrics:
 
     def get_nbc(self):
         """
-        Get the metric of 'neuron boundary coverage' :math` NBC =（|UpperCornerNeuron|
-        + |LowerCornerNeuron|）/(2*|N|)`, where :math`|N|` is the number of neurons,
+        Get the metric of 'neuron boundary coverage' :math:`NBC = (|UpperCornerNeuron|
+        + |LowerCornerNeuron|)/(2*|N|)`, where :math`|N|` is the number of neurons,
         NBC refers to the proportion of neurons whose neurons output value in
         the test dataset exceeds the upper and lower bounds of the corresponding
         neurons output value in the training dataset.
@@ -192,7 +192,7 @@ class ModelCoverageMetrics:
     def get_snac(self):
         """
         Get the metric of 'strong neuron activation coverage'.
-         :math` SNAC =|UpperCornerNeuron| / |N|`. SNAC refers to the proportion
+        :math:`SNAC = |UpperCornerNeuron|/|N|`. SNAC refers to the proportion
         of neurons whose neurons output value in the test set exceeds the upper
         bounds of the corresponding neurons output value in the training set.