doc: pretty-formats: separate parameters from placeholders

doc: pretty-formats: separate parameters from placeholders

Commit a57523428b4 (pretty: support padding placeholders, %< %> and %><,
2013-04-19) introduced columnated place holders. These placeholders
can be confusing as they contain `<` and `>` characters as part
of their placeholders adjacent to the `<N>` parameters.

Add spaces either side of the `<N>` parameters in the title line.
The code (strtol) will consume any spaces around the number values
(assuming they are passed as a quoted string with spaces).
Note that the spaces are optional.

Subsequent commits will clarify other confusions.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>