tectonic/xetex-linebreak.c: fix mistake introduced in a2e6a3e

Addresses tectonic-typesetting#617.

Author: pkgw

tectonic/xetex-linebreak.c

@@ -1992,7 +1992,7 @@ try_break(int32_t pi, small_number break_type)
                 cur_active_width[6] += DELTA_NODE_dshrink(r);
                 DELTA_NODE_dwidth(prev_r) += DELTA_NODE_dwidth(r);
                 DELTA_NODE_dstretch0(prev_r) += DELTA_NODE_dstretch0(r);
-                DELTA_NODE_dstretch2(prev_r) += DELTA_NODE_dstretch1(r);
+                DELTA_NODE_dstretch1(prev_r) += DELTA_NODE_dstretch1(r);
                 DELTA_NODE_dstretch2(prev_r) += DELTA_NODE_dstretch2(r);
                 DELTA_NODE_dstretch3(prev_r) += DELTA_NODE_dstretch3(r);
                 DELTA_NODE_dshrink(prev_r) += DELTA_NODE_dshrink(r);