Revert mermaid_config.js prettier-ignore comment (just-the-docs#1189)

I introduced a bug in just-the-docs#1172 --- there's an implicit requirement that the line in `mermaid_config.js` has either an expression or the start of a code block; a comment (and then a newline) is invalid.

(to maintain the ignore behaviour, I've instead moved it to the `.prettierignore`)

This one's on me -- I hadn't realized that the `prettier-ignore` affected this file. Given the relative urgency of this, going to merge this in.

Closes just-the-docs#1188.

Author: mattxwang

.prettierignore

@@ -8,3 +8,4 @@ assets/js/search-data.json
 assets/js/zzzz-search-data.json
 assets/js/just-the-docs.js
 *.md
+_includes/mermaid_config.js

_includes/mermaid_config.js

@@ -1,2 +1 @@
-// prettier-ignore
 {}