Code fence handling breaks if ~~~ appears in the fence

[KSR-Yasuda]

Text like below will break code fence.

# Heading 1

## Heading 1-1

```text
~~~hoge~~~
```

# Heading 2

## Heading 2-1

```
~~~piyo~~~
```

# Heading 3

## Heading 3-1

```text
hogefugapiyo
    ~~~~
```

# Heading 4

## Heading 4-1

Fixed the check pattern, and fence ending check to compare with the fence beginning string.

:Toc command and text folding do work.

Can anyone fix syntax syntax/markdown.vim?

[KSR-Yasuda]

Thank you for merging!