gas/doc: adjust an @xref
Old doc tools warn about there not being a . or , following; satisfy
those tools by shortening the line and adding a full stop.
diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index 3501744..0b4109e 100644
--- a/gas/doc/as.texi
+++ b/gas/doc/as.texi
@@ -4788,7 +4788,7 @@
@item No passing arguments to macros based upon keyword assignment.
In altmacro mode arguments cannot be passed to macros by keyword assignment.
-See @xref{altmacro-keyword-arguments} for more details
+See @xref{altmacro-keyword-arguments}.
@end ftable
@node Ascii
