Update import statement for fontkit in README.md

[kuo-hm]

This commit updates the import statement for the fontkit library in the README.md file.

Previously, the import statement was written as import fontkit from '@pdf-lib/fontkit';. However, this might cause issues if the library does not have a default export.

To ensure compatibility with different module systems, the import statement has been changed to import * as fontkit from '@pdf-lib/fontkit';. This syntax imports all exports from the fontkit library into an object named fontkit, which can then be used to access the library's features.