Fix matchInContainer false positives

The function should match only the beginnings of lines.

For example, see this testcase:
```
:::spoiler
here is a :::
:::
:::
```
The last line should be completed.

Without this patch, the third line is completed and the last is not.

Signed-off-by: Tamotsu Takahashi <ttakah@gmail.com>

public/js/index.js

@@ -3137,7 +3137,7 @@ function checkInContainerSyntax () {
 
 function matchInContainer (text) {
   var match
-  match = text.match(/:{3,}/g)
+  match = text.match(/(^|\n):::/gm)
   if (match && match.length % 2) {
     return true
   } else {