Format Role Names

[braughtg]

Change the role names "manager," "worker," and "guest" to be formatted in the same way that "admin" is. They should be formatted using back-ticks (`) for code instead of (_) for italics to match "admin" in the second bullet point under "FarmData2 Module Structure" in the README.md file in the farmdata2_modules/fd2_tabs/fd2_example directory.

[burdged]

Darci