Minor improvements to Rd lexer.
Minor improvements to Rd lexer.

kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,dbrgn/pygments-mirror,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror,kirbyfan64/pygments-unofficial,dbrgn/pygments-mirror