obsolete: let N denote number of obsoleted changestes in a marker

The number of obsoleted changesets is referred to as N later in the
docstring.

mercurial/obsolete.py

@@ -66,7 +66,7 b' The file starts with a version header:'
 
 The header is followed by the markers. Each marker is made of:
 
-- 1 unsigned byte: number of new changesets "R", can be zero.
+- 1 unsigned byte: number of new changesets "N", can be zero.
 
 - 1 unsigned 32-bits integer: metadata size "M" in bytes.