I had this problem before and suggested that dictionary editing was added to Joplin as it is not as simple as just editing a text file.
On Windows the file is located at:
C:\Users\<username>\AppData\Roaming\@joplin\app-desktop\Custom Dictionary.txt
IIRC there is an @joplin equivalent on Linux.
Edit: /home/<username>/.config/@joplin/app-desktop/Custom Dictionary.txt
However Custom Dictionary.txt is protected by an MD5 hash and if the hash does not match, the last known good backup file the spell-checker keeps (Custom Dictionary.txt.backup) is promoted to primary. In effect this replaces the edited dictionary with one that contains the word you do not want, unless the "wrong" word was the very last one you added.
The last line of the file looks something like:
checksum_v1 = b90fcb2494abf01cc226588851123e4f
You will have to delete that line and the wrong word. Then obtain the MD5 hash for the new text file ensuring that the cursor when you save the file is on an empty line immediately after the last word and that empty line is the end of the file.
Then edit the file again add the new MD5 hash value as:
checksum_v1 = <new checksum>
On that last line immediately after the last dictionary word.
If anyone has an easier method I would be glad to hear it!