Update rule B005 docs (#8028)
## Summary
Rule B005 of flake8-bugbear docs has a typo in one of the examples that
leads to a confusion in the correctness of `.strip()` method
```python
# Wrong output (used in docs)
"text.txt".strip(".txt") # "ex"
# Correct output
"text.txt".strip(".txt") # "e"
```
