CodingGuidelines: do not call the conditional statement "if()"

The point immediately before it is about having SP after the control
keyword.  Spell it out as 'an "if" statement' instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -194,7 +194,7 @@ For C programs:
    of "else if" statements, it can make sense to add braces to
    single line blocks.
 
- - We try to avoid assignments inside if().
+ - We try to avoid assignments in the condition of an "if" statement.
 
  - Try to make your code understandable.  You may put comments
    in, but comments invariably tend to stale out when the code