Comment spelling fix.

bash_completion

@@ -1284,7 +1284,7 @@ _realcommand()
     }
 }
 
-# This function returns the first arugment, excluding options
+# This function returns the first argument, excluding options
 # @param $1 chars  Characters out of $COMP_WORDBREAKS which should
 #     NOT be considered word breaks. See __reassemble_comp_words_by_ref.
 _get_first_arg()