add comma in sentence

Corrected punctuation in the explanation of itemization.

Author: schultzdavid

doc/Language/contexts.rakudoc

@@ -241,7 +241,7 @@ $(1,2,3).elems.say;          # OUTPUT: «3␤»
 say (1,2,3).item[2];         # OUTPUT: «3␤»
 say $( key => 'value')<key>; # OUTPUT: «value␤»
 
-Itemization I<containerizes> values in a data structure keeping them, for
+Itemization I<containerizes> values in a data structure, keeping them, for
 instance, from being flattened into the surrounding list or data structure:
 
 =for code