templatekw: specify plural form of instability

Follows up 40739aef97f7.

mercurial/templatekw.py

@@ -783,7 +783,8 b' def showinstabilities(**args):'
     (EXPERIMENTAL)
     """
     args = pycompat.byteskwargs(args)
-    return showlist('instability', args['ctx'].instabilities(), args)
+    return showlist('instability', args['ctx'].instabilities(), args,
+                    plural='instabilities')
 
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = keywords.values()