Replace "nodejs-ex" by "nodejs-exemple" in README file

[frederic-danna]

(should be tagged with the "kind/documentation" label)

In then README file, the string "nodejs-ex" must be replaced by "nodejs-exemple".

For instance, in the "Build the app" section of this file:

• the expected output of os status command: at least 3 occurrences
• 1 occurrence in the "start-build" command: oc start-build nodejs-ex --follow must be replaced by oc start-build nodejs-example --follow