docs: Fix errors in BaseModelView docstring

[XD-DENG]

Description

Purely docstring changes. No touch on actual code.

These two docstring errors are affecting:

• https://flask-appbuilder.readthedocs.io/en/latest/api.html#flask_appbuilder.baseviews.BaseModelView.search_form_extra_fields
  and
• https://flask-appbuilder.readthedocs.io/en/latest/api.html?highlight=base_filters#flask_appbuilder.baseviews.BaseModelView.search_form_query_rel_fields

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1591 (a9e06db) into master (d04ec3c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1591   +/-   ##
=======================================
  Coverage   75.44%   75.44%           
=======================================
  Files          54       54           
  Lines        7869     7869           
=======================================
  Hits         5937     5937           
  Misses       1932     1932           

Flag	Coverage Δ
python	75.44% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/baseviews.py	89.86% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d04ec3c...a9e06db. Read the comment docs.