Update checkrelease script to check readme.md instead of .txt

[btashton]

Summary

When the readme was converted to markdown the extension to look for in in the release verification script was not updated and would fail like this:

Checking apache-nuttx-10.0.0-incubating.tar.gz for required files:
 - README.txt file not present.

This corrects the filename.

Impact

The release checking script now passes.

Testing

Here it is running against the latest staged RC

❯ ./tools/checkrelease.sh --release 10.0.0-RC0
<<TRIM>>
Checking apache-nuttx-10.0.0-incubating.tar.gz sha512...
 OK: apache-nuttx-10.0.0-incubating.tar.gz sha512 hash matches.

Checking apache-nuttx-10.0.0-incubating.tar.gz GPG signature:
<<TRIM>>
Primary key fingerprint: 3554 D784 58CE B695 4B02  0E12 E1B6 E30D B05D 6280
     Subkey fingerprint: 66C4 832A 165E CC93 5489  5A20 9750 ED7E 692B 99E2
 OK: apache-nuttx-10.0.0-incubating.tar.gz gpg signature matches.

Checking apache-nuttx-10.0.0-incubating.tar.gz for required files:
 OK: all required files exist in nuttx.

Checking apache-nuttx-apps-10.0.0-incubating.tar.gz sha512...
 OK: apache-nuttx-apps-10.0.0-incubating.tar.gz sha512 hash matches.

Checking apache-nuttx-apps-10.0.0-incubating.tar.gz GPG signature:
<<TRIM>>
 OK: apache-nuttx-apps-10.0.0-incubating.tar.gz gpg signature matches.

Checking apache-nuttx-apps-10.0.0-incubating.tar.gz for required files:
 OK: all required files exist in apps.

Trying to build nuttx sim:nsh...
 OK: we were able to build sim:nsh.

[Ouss4]

Do you plan to backport this one? I don't think we should cancel the current RC for a change in tools/.

[btashton]

Do you plan to backport this one? I don't think we should cancel the current RC for a change in tools/.

I was not planning on it, if we do a RC1 we can backport it in.