Fix typo in @cmd docstring (#56664)

I'm not sure what `` `cmd` `` could refer to, but it would make sense to
refer to `` `str` `` in this case. I'm assuming it's a typo.

Author: LilithHafner

base/cmd.jl

@@ -491,7 +491,7 @@ end
 """
     @cmd str
 
-Similar to `cmd`, generate a `Cmd` from the `str` string which represents the shell command(s) to be executed.
+Similar to ``` `str` ```, generate a `Cmd` from the `str` string which represents the shell command(s) to be executed.
 The [`Cmd`](@ref) object can be run as a process and can outlive the spawning julia process (see `Cmd` for more).
 
 # Examples