Add entries for URL encoding and decoding

manuscript/chapter6.txt

@@ -21,7 +21,7 @@ $ hello_world="value"
 $ var="world"
 
 # Declare a nameref.
-$ declare -n "ref=hello_$var"
+$ declare -n ref=hello_$var
 
 $ printf '%s\n' "$ref"
 value
@@ -31,7 +31,7 @@ value
 
 ```shell
 $ var="world"
-$ declare hello_$var=value
+$ declare "hello_$var=value"
 $ printf '%s\n' "$hello_world"
 value
 ```