Fix antlr#3508: Document the $parser attribute and its use in target-…

…agnostic grammars.

doc/python-target.md

@@ -124,28 +124,28 @@ Unfortunately, this is not portable, as Java and Python (and other target langua
 
 Thanks to the above, you should be able to rewrite the above semantic predicate as follows:
 
-File MyParser.g4:
+File `MyGrammarParser.g4`:
 ```
-options { superClass = BaseParser; }
+options { superClass = MyGrammarBaseParser; }
 ...
 ID {$parser.isEqualText($text,"test")}?
 ```
 
-File BaseParser.py:
+File `MyGrammarBaseParser.py`:
 ```python
 from antlr4 import *
 
-class BaseParser(Parser):
+class MyGrammarBaseParser(Parser):
 
    def isEqualText(a, b):
       return a is b
 ```
 
-File BaseParser.java
+File `MyGrammarBaseParser.java`:
 ```java
 import org.antlr.v4.runtime.*;
 
-public abstract class BaseParser extends Parser {
+public abstract class MyGrammarBaseParser extends Parser {
 
    public static boolean isEqualText(a, b) {
       return a.equals(b);