Fix spelling error with word staff

src/main/asciidoc/models/design/staff.puml

@@ -60,7 +60,7 @@ package staff {
 StaffRepository *-- Staff : -staff
 StaffController --> StaffRepository : -staffRepository
 StaffDataInitializer --> StaffRepository : -staffRepository
-StaffRepository --> Set : -staffs
+StaffRepository --> Set : -staff
 StaffRepository --> HashSet : use
 StaffRepository --> Collection : use
 StaffRepository --> Optional : use

src/main/asciidoc/models/design/systemContextDiagram.puml

@@ -8,6 +8,6 @@ usecase (Cateringservice\n[Softwaresystem]\nan online software for planning a ca
 
 UU ---> Use: login as customer\nor administrator,\nregister as new customer,\nview catalog
 K ---> Use: plan a \ncatering event
-A ---> Use: remove customers,\nmanage inventory/staffs
+A ---> Use: remove customers,\nmanage inventory/staff
 
 @enduml

src/main/asciidoc/models/design/topLevelArchitecture.puml

@@ -42,7 +42,7 @@ Kunde ----> UC3 : "**plan a catering event**\n[HTTPS]"
 Kunde ---- UC5 : "**delete account, change personal data**\n[HTTPS]"
 
 Boss ----> UC3 : "**view orders**\n[HTTPS]"
-Boss ----> UC4 : "**manage inventory/staffs**\n[HTTPS]"
+Boss ----> UC4 : "**manage inventory/staff**\n[HTTPS]"
 Boss ----> UC2 : "**modify catalog**\n[HTTPS]"
 
 UC2 ----> DB : "**read and write catalog data**\n[JPA]"