exim documentation: minor touchup

README.exim.md

     
     begin routers
     
-    srs0_return:
+    srs_return:
       caseful_local_part
       domains = SRS_DOMAIN
       driver = redirect
     
     begin routers
     
-    srs0_return:
+    srs_return:
       caseful_local_part
       domains = SRS_DOMAIN
       local_part_prefix = srs0= : srs1=