Issue with english WFST inverse text normalizer and "per cent"
**Describe the bug**

The english WFST inverse text normalizer fails to correctly tag "per cent". 

**Steps/Code to reproduce bug**

```
from nemo_text_processing.inverse_text_normalization.inverse_normalize import InverseNormalizer
inverse_normalizer = InverseNormalizer(lang='en')

spoken = "The share price of BHP increased by eleven per cent"
un_normalized = inverse_normalizer.inverse_normalize(spoken, verbose=False)
print(un_normalized)
```

**Expected behavior**

I expect "The share price of BHP increased by 11 %". 

The actual output behavior is an unchanged input - "The share price of BHP increased by eleven per cent"

NB: If I change the input to "... per cents", then I do get the expected output. There's an issue with how the plural / singular forms are handled in this case 


