docs: swap descriptions for [extname] and [ext]

[tivac]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

While working on plugin stuff I noticed that I was using [extname] and seeing a leading period in the result, which the docs said I shouldn't. I went and looked at the rollup source for this functionality:

rollup/src/utils/assetHooks.ts

Lines 34 to 37 in 00d7521

	case 'extname':
	return extname(asset.name);
	case 'ext':
	return extname(asset.name).substr(1);

Which sure makes it look like the docs were wrong. So I swapped [extname] and [ext] so now they line up correctly based on what they do.

[lukastaegert]

Thanks a lot! Yes, extname was meant to match up with path.extname, got a little mixed up in the docs 🙄