fix postinstallcmds and clarify download instructions for netMHCpan-4.0a

[lexming]

(created using eb --new-pr)

[lexming]

Test report by @lexming
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
login2.cerberus.os - Linux centos linux 7.7.1908, Intel(R) Xeon(R) Gold 6126 CPU @ 2.60GHz, Python 2.7.5
See https://gist.github.com/6675d4754eb943c17af702e19a7df793 for a full test report.

[lexming]

@boegelbot: please test @ generoso

[boegelbot]

@lexming: Request for testing this PR well received on login1

PR test command 'EB_PR=9740 EB_ARGS= EB_CONTAINER= /opt/software/slurm/bin/sbatch --job-name test_PR_9740 --ntasks=4 ~/boegelbot/eb_from_pr_upload_generoso.sh' executed!

• exit code: 0
• output:

Submitted batch job 10020

Test results coming soon (I hope)...

Details

- notification for comment with ID 1386701651 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in total)
cns1 - Linux Rocky Linux 8.5, x86_64, Intel(R) Xeon(R) CPU E5-2667 v3 @ 3.20GHz (haswell), Python 3.6.8
See https://gist.github.com/1fee6622214d4a7dcc246a0f075ae5c9 for a full test report.

[smoors]

lgtm

[smoors]

Going in, thanks @lexming!