Fix layout strings that cause a make error

We do not currently handle UTF-8 characters in our layout strings.
Replace dashes with simple ASCII dashes to avoid the following
error:

  msguniq -o lyx.po
  msguniq: input file '-' doesn't contain a header entry with a
  charset specification

This commit amends the recent commit b4dcad83.

lib/layouts/aastex6.layout

@@ -100,12 +100,12 @@ Style Appendix
 End
 
 Style TableRefs
-	LabelString	"References—"
+	LabelString	"References-"
 	LabelFont
 	  Series	Bold
 	EndFont
 End
 
 Style TableComments
-	LabelString	"Note—"
+	LabelString	"Note-"
 End