Fixing the readme file for align_trim usage

[Rohit-Satyam]

Description of the bug

Hi @jameshadfield

Thanks for separating this module as it did come handy for me lately. I think the correct command to run aligntrim is via align_trim not aligntrim as mentioned in the README file. Your input also make use of --bam tag whereas your tool actually has a --samfile tag

Command used and terminal output

aligntrim primers.bed --bamfile input.bam --output trimmed.bam