Spoiler block captions containing a period not immediately followed by white space are rendered incorrectly

### Is this a hackmd.io issue?

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

### What's the problem

#### Current behaviour

When using a `:::spoiler` block with a caption that contains a `.`, everything from the `.` up to and including the next white space in the caption seems to be ignored for rendering.


[See this example note, displaying the issue in three different ways.](https://hackmd.io/MxeHSzXuSOeX-Zwsgukz2Q?both)

Steps to reproduce:

1. Create a note containing a spoiler block in which the caption contains a period not immediately followed by white space.

#### Expected behaviour

The caption is rendered faithfully, as it would be in the body of a paragraph.

#### Environment

I use HackMD on:

**Desktop**
- OS: Linux (Ubuntu 22.04)
- Browser: Chrome
- Browser Version: 126.0.6478.126