Replace django-admin.py with django-admin

Running
```
django-admin.py startproject superlists .
```
opens the file `django-admin.py`

whereas running
```
django-admin startproject superlists . 
```
runs as expected

using Django v1.11.25

Author: rdmolony

chapter_01.asciidoc

@@ -151,7 +151,7 @@ Django provides a little command-line tool for this:
 
 [subs="specialcharacters,quotes"]
 ----
-$ *django-admin.py startproject superlists .*
+$ *django-admin startproject superlists .*
 ----
 
 Don't forget that "." at the end; it's important!