Miscellaneous improvements

manuscript/chapter3.txt

@@ -41,12 +41,12 @@ arr=(apples oranges tomatoes)
 
 # Elements and index.
 for i in "${!arr[@]}"; do
-    printf '%s\n' "${arr[$i]}"
+    printf '%s\n' "${arr[i]}"
 done
 
 # Alternative method.
 for ((i=0;i<${#arr[@]};i++)); do
-    printf '%s\n' "${arr[$i]}"
+    printf '%s\n' "${arr[i]}"
 done
 ```