[DOC] Add missing escape

Before:
```
* + corresponds to $+, which contains last capture group.
```

After:
```
* \+ corresponds to $+, which contains last capture group.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/string.rb b/string.rb
index b410e09395..be10b407b0 100644
--- a/string.rb
+++ b/string.rb
@@ -90,7 +90,7 @@
 #    which contains string after match.
 #  - <tt>\`</tt> corresponds to <tt>$`</tt>,
 #    which contains string before match.
-#  - <tt>\+</tt> corresponds to <tt>$+</tt>,
+#  - <tt>\\+</tt> corresponds to <tt>$+</tt>,
 #    which contains last capture group.
 #
 #  See rdoc-ref:regexp.rdoc for details.