Fix webmanifest filename
There was a mismatch between the webmanifest's file name and the reference in index.html (https://gitlab.com/particify/dev/foss/arsnova-webclient/-/blob/d744c25fdb52477d317d24087c58618a1f901fd1/src/index.html#L25).
Regression-MR: !1832 (merged)
Edited by Daniel Gerhardt