fix grammer

Co-authored-by: Nate Baer <[email protected]>
Signed-off-by: Alejandro Reyes <[email protected]>

README.md

@@ -670,7 +670,7 @@ Output:
 
 
 Both the `field` and the global Blueprinter Configuration supports `:if` and `:unless` options that can be used to serialize fields conditionally.
-By default field-level conditions override global conditions. By enabling `roll_up_conditions`, BOTH field-level and global conditions must be met in order for field to be serialized.
+By default, field-level conditions override global conditions. By enabling `roll_up_conditions`, BOTH field-level and global conditions must be met in order for the field to be serialized.
 
 ### Global Config Setting - if and unless