Remove excess whitespace from doc strings of some commands

I've noticed that doc strings of some commands, like "set cwd"
and  "set inferior-tty", have some excess whitespace, which
makes them display with unexpected indentation, at least in a
Windows command prompt window.  This patch fixes that.

* gdb/linux-nat.c (_initialize_linux_nat):
* gdb/riscv-tdep.c (riscv_insn):
* gdb/top.c (quit_force):
* gdb/infcmd.c (_initialize_infcmd): Remove excess whitespace.