use conventional whitespace in interactive example

Doc/lib/libdircache.tex

@@ -32,8 +32,8 @@ in place, a \character{/} to each path which points to a directory.
 
 \begin{verbatim}
 >>> import dircache
->>> a=dircache.listdir('/')
->>> a=a[:] # Copy the return value so we can change 'a'
+>>> a = dircache.listdir('/')
+>>> a = a[:] # Copy the return value so we can change 'a'
 >>> a
 ['bin', 'boot', 'cdrom', 'dev', 'etc', 'floppy', 'home', 'initrd', 'lib', 'lost+
 found', 'mnt', 'proc', 'root', 'sbin', 'tmp', 'usr', 'var', 'vmlinuz']