fix: correct a subtitle

[hanyujie2002]

This subtitle was supposed to be modified without doing more research in #26350

content/files/en-us/learn/tools_and_testing/cross_browser_testing/javascript/index.md

Lines 222 to 229 in e6fcda9

	## Cross-browser JavaScript problems
	In this section, we'll look at some of the more common cross-browser JavaScript problems. We'll break this down into:
	- Using modern core JavaScript features
	- Using modern Web API features
	- Using bad browser sniffing code
	- Performance problems

However, the subtitle have correspondence in line 228. The map between the problem list and the subtitles will be broken with the subtitle be modified

[github-actions]

Preview URLs

• /en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/JavaScript

[Josh-Cena]

If someone changes the section title, they mean to change the section title. The bug is in what they didn't change, not in what they changed. Please change the anchor links that are outdated instead.

[Josh-Cena]

I believe the current heading is correct. "Using bad sniffing code" is a common problem and the solution is to "don't browser sniff", because it's almost impossible to write "good sniffing code".