Adapt to environment variable naming to convention used in manual

[tb-rti]

Summary

Adapt names of environment variables in cloud discovery service example to the names used in the manual.

Details and comments

Fixes #521

Checks

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[lulivi]

Hi @tb-rti, can you fix this lint issues?

examples/connext_dds/real_time_wan_transport/README.md:131:81 MD013/line-length Line length [Expected: 80; Actual: 95]
examples/connext_dds/real_time_wan_transport/README.md:132:81 MD013/line-length Line length [Expected: 80; Actual: 88]
examples/connext_dds/real_time_wan_transport/README.md:156:81 MD013/line-length Line length [Expected: 80; Actual: 106]