[LyX/master] Fix string subst

[Juergen Spitzmueller]

commit 5cc191ddeef716a955e9a86364eaa61ebe43fed3
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Thu Aug 27 11:52:23 2020 +0200

    Fix string subst
---
 src/insets/InsetCitation.cpp |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/insets/InsetCitation.cpp b/src/insets/InsetCitation.cpp
index 0c4b3fe..dc4bb2d 100644
--- a/src/insets/InsetCitation.cpp
+++ b/src/insets/InsetCitation.cpp
@@ -211,7 +211,7 @@ void InsetCitation::openCitation()
 				       from_ascii(lyxrc.citation_search_pattern));
 		// some cleanup: commas and " and ", as used in name lists,
 		// are not expected in file names
-		titledata = subst(titledata, ',', char());
+		titledata = subst(titledata, from_ascii(","), docstring());
 		titledata = subst(titledata, from_ascii(" and "), from_ascii(" "));
 		bi.getLocators(kvar, doi, url, file);
 		LYXERR(Debug::INSETS, "Locators: doi:" << doi << " url:"