Fix translation for "Observer"

[eduardomozart]

The correct string is 'Observer' instead of 'Watcher'.

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes # (issue number, if applicable)
• Here is a brief description of what this PR does: The correct string is 'Observer' instead of 'Watcher' in GLPI translation files (*.po).

Screenshots (if appropriate):

As can be seen in screenshot below, the word "Watcher" and "Watcher group" isn't being translated to PT-BR.