docs: use f-strings instead format (#585)

In the documentation use f-strings in the examples.

docs/cli.rst

@@ -23,7 +23,7 @@ might look like this::
         verbose = cli.Flag(["v", "verbose"], help = "If given, I will be very talkative")
 
         def main(self, filename):
-            print("I will now read {0}".format(filename))
+            print(f"I will now read {filename}")
             if self.verbose:
                 print("Yadda " * 200)
 
@@ -492,7 +492,7 @@ attached to the root application using the ``subcommand`` decorator ::
 
         def main(self, *args):
             if args:
-                print("Unknown command {0!r}".format(args[0]))
+                print(f"Unknown command {args[0]}")
                 return 1   # error exit code
             if not self.nested_command:           # will be ``None`` if no sub-command follows
                 print("No command given")

docs/colorlib.rst

@@ -246,9 +246,9 @@ For example, if you wanted to create an HTMLStyle and HTMLcolors, you could do::
             result = ''
 
             if self.bg and not self.bg.reset:
-                result += '<span style="background-color: {0}">'.format(self.bg.hex_code)
+                result += f'<span style="background-color: {self.bg.hex_code}">'
             if self.fg and not self.fg.reset:
-                result += '<font color="{0}">'.format(self.fg.hex_code)
+                result += f'<font color="{self.fg.hex_code}">'
             for attr in sorted(self.attributes):
                 if self.attributes[attr]:
                     result += '<' + self.attribute_names[attr] + '>'