Improve suggestion for broken_footnote lint

Author: GuillaumeGomez

src/librustdoc/passes/lint/footnotes.rs

@@ -87,7 +87,7 @@ pub(crate) fn visit_item(cx: &DocContext<'_>, item: &Item, hir_id: HirId, dox: &
                 lint.span_suggestion(
                     ref_span.shrink_to_lo(),
                     "if it should not be a footnote, escape it",
-                    "\\",
+                    format!("\\{}", &dox[span]),
                     Applicability::MaybeIncorrect,
                 );
             }),

tests/rustdoc-ui/lints/broken-footnote.stderr

@@ -4,7 +4,7 @@ error: no footnote definition matching this footnote
 LL | //! [^*] special characters can appear within footnote references
    |     -^^^
    |     |
-   |     help: if it should not be a footnote, escape it: `\`
+   |     help: if it should not be a footnote, escape it: `\[^*]`
    |
 note: the lint level is defined here
   --> $DIR/broken-footnote.rs:1:9
@@ -18,23 +18,23 @@ error: no footnote definition matching this footnote
 LL | //! Footnote referenced [^1]. And [^2]. And [^bla].
    |                                   -^^^
    |                                   |
-   |                                   help: if it should not be a footnote, escape it: `\`
+   |                                   help: if it should not be a footnote, escape it: `\[^2]`
 
 error: no footnote definition matching this footnote
   --> $DIR/broken-footnote.rs:3:45
    |
 LL | //! Footnote referenced [^1]. And [^2]. And [^bla].
    |                                             -^^^^^
    |                                             |
-   |                                             help: if it should not be a footnote, escape it: `\`
+   |                                             help: if it should not be a footnote, escape it: `\[^bla]`
 
 error: no footnote definition matching this footnote
   --> $DIR/broken-footnote.rs:16:5
    |
 LL | //! [^\_] so can escaped characters
    |     -^^^^
    |     |
-   |     help: if it should not be a footnote, escape it: `\`
+   |     help: if it should not be a footnote, escape it: `\[^\_]`
 
 error: aborting due to 4 previous errors