Comments are broken when an abbreviation is present on the commented line

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### What's the problem

Sometimes, comments are linked to multiple portions of the document, instead of being linked to the portion selected by the user.

#### Current behaviour


[Take a look at this demo](https://hackmd.io/qCMlLXlBTReZFdSHM-K_Xw?view)

To reproduce the issue, create a new document with the following content and try commenting different lines:

```markdown
This is a line without acronyms and without comments.

This is a line without acronyms with working comments.

This is a line with an acronym, with broken comments. Did you know who invented HTTP?

*[HTTP]: HyperText Transfer Protocol
```

#### Expected behaviour

The comment should be linked only to the selected portion of the document. No other portions of the document should be highlighted or linked to the original comment.

#### Environment

I use HackMD on the web.