Fix typo in cdt package to install on conda-forge documentation

[traversaro]

expat-cos6-x86_64 should be expat-cos7-x86_64.

See:

• In conda deps do not install mesalib, switch to cos7 and switch back to YARP master in Unstable branches #926 the original pr
• Dropping CentOS 6 & Moving to CentOS 7 conda-forge/conda-forge.github.io#1436 the indication that probably conda-forge as an whole will be moving to cos7 soon, that actually give even more sense to In conda deps do not install mesalib, switch to cos7 and switch back to YARP master in Unstable branches #926

[HosameldinMohamed]

I am not sure which policy you have for updating the changelog file.
But this to remind you about that 😀

[traversaro]

I am not sure which policy you have for updating the changelog file. But this to remind you about that 😀

Thanks! To be honest I tend not to put small changes in the superbuild changelog, but thanks for the remainder!