Documentation: Use static default text in BigText demo

[penguinolog]

Version based demo text causes screenshot re-generation.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 6348502664

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.38%

---

Totals
Change from base Build 6341349895:	0.0%
Covered Lines:	7514
Relevant Lines:	10286

---

💛 - Coveralls