fix: avoid leaking wrapper space from doc comment code block formatting

src/lib.rs

@@ -387,6 +387,8 @@ fn format_code_block(
     let block_len = formatted
         .snippet
         .rfind('}')
+        // trim and trailing whitespace we added, e.g. if we format to fn main { some_func(); }
+        .map(|i| formatted.snippet[..i].strip_suffix(' ').map_or(i, str::len))
         .unwrap_or_else(|| formatted.snippet.len());
 
     // It's possible that `block_len < FN_MAIN_PREFIX.len()`. This can happen if the code block was

tests/target/issue-6832.rs

@@ -0,0 +1,7 @@
+// rustfmt-format_code_in_doc_comments: true
+// rustfmt-fn_single_line: true
+
+/// ```rust
+/// let font = String::from("hello");
+/// ```
+fn main() {}