[llvm-cov] - Improve llvm-cov error message

Summary:

When giving the following command:
% llvm-cov report -instr-profile=default.profraw

llvm-cov will give the following error message:

>llvm-cov report: Not enough positional command line arguments specified!
>Must specify at least 1 positional arguments: See: orbis-llvm-cov report -help

This patch changes the error message from  '1 positional arguments'
to '1 positional argument'.

Differential Revision: https://reviews.llvm.org/D22621

llvm-svn: 276404