patch 8.2.1168: wrong method argument for appendbufline()

Problem:    Wrong method argument for appendbufline().
Solution:   Use FEARG_3.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 86a676c..3f38fa7 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -466,7 +466,7 @@
     {"add",		2, 2, FEARG_1,	  ret_first_arg, f_add},
     {"and",		2, 2, FEARG_1,	  ret_number,	f_and},
     {"append",		2, 2, FEARG_2,	  ret_number,	f_append},
-    {"appendbufline",	3, 3, FEARG_2,	  ret_number,	f_appendbufline},
+    {"appendbufline",	3, 3, FEARG_3,	  ret_number,	f_appendbufline},
     {"argc",		0, 1, 0,	  ret_number,	f_argc},
     {"argidx",		0, 0, 0,	  ret_number,	f_argidx},
     {"arglistid",	0, 2, 0,	  ret_number,	f_arglistid},