[5.0.x] Refs #34140 -- Fixed blacken-docs pre-commit configuration.

Missed in 6015bab80e28aef2669f6fac53423aa65f70cb08. The default
blacken-docs hook definition does not apply to .txt files, which the
Django documentation uses. This commit overrides that definition to
point blacken-docs at the appropriate files.
Backport of d89a465e62ad876cc7f1332d1712700cb81f3995 from main

.pre-commit-config.yaml

@@ -10,6 +10,7 @@ repos:
       - id: blacken-docs
         additional_dependencies:
         - black==23.10.0
+        files: 'docs/.*\.txt$'
   - repo: https://github.com/PyCQA/isort
     rev: 5.12.0
     hooks: