Change email string to username

app/src/main/res/values/strings.xml

@@ -2,8 +2,8 @@
     <string name="app_name">Canteeneo</string>
 
     <!-- Strings related to login -->
-    <string name="prompt_email">Email</string>
-    <string name="prompt_password">Password (optional)</string>
+    <string name="prompt_username">Username</string>
+    <string name="prompt_password">Password</string>
     <string name="action_sign_in">Sign in or register</string>
     <string name="action_sign_in_short">Sign in</string>
     <string name="error_invalid_email">This email address is invalid</string>