Invalid Annotation in Test Member

[oleibman]

PhpUnit cannot parse the @covers lines in FormulaTest; they result in warnings in Coverage and Deploy tests. This PR fixes them; no change log entry should be needed.

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

Checklist:

• [x] I have run composer run-script check --timeout=0 and no errors were reported
• [x] The new code is covered by unit tests (check build/coverage for coverage report)
• [ ] I have updated the documentation to describe the changes

[coveralls]

coverage: 97.217%. remained the same when pulling f92acf10f183548ad9a978ac3e2cf9218469ff02 on oleibman:invalidcov into 41cf4ebddb6457ab20552630c27e33e2715dcfe9 on PHPOffice:master.