Fix "File: " skipping when building the elements of the imenu

We shouldn't be using `skip-chars-forward' here as this function skips
over any _character set_ passed as argument, not just a string.

Without this patch, for instance entries like:

  File: lol/foo.bar

Will be added to the candidates list as "ol/foo.bar" as ?l is in the
character set "File: ".

Instead, just skips 6 chars forward.

rg-result.el

@@ -777,7 +777,7 @@ previous file with grouped matches."
               (while (setq nextfile (rg-navigate-file-message nextfile nil 1))
                 (save-excursion
                   (goto-char nextfile)
-                  (skip-chars-forward "File: ")
+                  (and (looking-at-p "^File: ") (forward-char 6))
                   (setq filepath (buffer-substring-no-properties (point) (line-end-position))))
                 (push (cons filepath nextfile) elements))
               (nreverse elements))))))