Fix typo and clarify input file argument in Interpreter help message

[Nokhrin]

Problem

The help message for org.antlr.v4.gui.Interpreter (invoked via antlr4-parse or directly via java -cp) contains a typo in the class name (Intrepreter) and uses the abstract term input-filename(s), which can be confusing for users trying to understand what kind of files the tool expects.

Additionally, the fallback message uses the word "rig", which is ambiguous in this context.

Solution

This PR updates the help message printed to stderr when the tool is invoked with insufficient arguments:

1. Fixes the typo: Intrepreter → Interpreter.
2. Replaces input-filename(s) with source-code-file(s) to explicitly indicate that the tool expects plain text source code files in the target language.
3. Updates the stdin fallback message to use the new terminology and replaces "rig" with "the tool".

Before

java org.antlr.v4.gui.Intrepreter [X.g4|XParser.g4 XLexer.g4] startRuleName
  [-tokens] [-tree] [-gui] [-encoding encodingname]
  [-trace] [-profile filename.csv] [input-filename(s)]
Omitting input-filename makes rig read from stdin.

After

java org.antlr.v4.gui.Interpreter [X.g4|XParser.g4 XLexer.g4] startRuleName
  [-tokens] [-tree] [-gui] [-encoding encodingname]
  [-trace] [-profile filename.csv] [source-code-file(s)]
Omitting source-code-file makes the tool read from stdin.

Verification

Run the interpreter with insufficient arguments to trigger the help message:

java -cp ~/projects/antlr4/tool/target/antlr4-4.13.3-SNAPSHOT-complete.jar org.antlr.v4.gui.Interpreter
# or
antlr4-parse

The output should match the "After" block above.