fdo#83037 fix autocorrect collisions of short and long ASCII arrows

Now single --> character sequences will be replaced with single
arrows again, not with a dash plus arrow:

--> -> →

Only the short variants will be replaced within words:

File->Open ->  File→ Open
File-->Open ->  File-→ Open

Change-Id: Idf58f96cd3299799bcb2be0f86c0ebe8937c70ae