Chore: fix a description of count --no-headers option

[Kanezoh]

related to #329

just fix a description of count --no-headers option.

$ ./target/debug/xsv count --help
Prints a count of the number of records in the CSV data.

Note that the count will not include the header row (unless --no-headers is
given).

Usage:
    xsv count [options] [<input>]

Common options:
    -h, --help             Display this message
    -n, --no-headers       When set, the first row will be included in
                           the count.
    -d, --delimiter <arg>  The field delimiter for reading CSV data.
                           Must be a single character. (default: ,)