Update indentation.md to fix a typo (scala#23505)

Closes scala#23492
[Cherry-picked 26d1794]

Author: bingchen-li

docs/_docs/reference/other-new-features/indentation.md

@@ -189,7 +189,7 @@ Define for an arbitrary sequence of tokens or non-terminals `TS`:
 
 ```ebnf
 :<<< TS >>>   ::=   ‘{’ TS ‘}’
-                |   <colon> <indent" TS <outdent>
+                |   <colon> <indent> TS <outdent>
 ```
 Then the grammar changes as follows:
 ```ebnf