fix grammar and style issues in man pages for stat related pages

This patch addresses grammatical errors and style inconsistencies in
the ifstat, lnstat, and rtacct man pages documentation.

lnstat Changes:
  - Capitalize "Linux" as proper noun in NAME and DESCRIPTION sections
  - Add missing period to -j/--json option description
  - Fix passive voice: "is being used" -> "is used"
  - Fix word form: "can not" -> "cannot"

rtacct Changes:
  - Fix date format: remove comma in "27 June, 2007" -> "27 June 2007"
  - Remove period from NAME section per man page convention
  - Capitalize SNMP acronym (2 instances)
  - Add missing periods to -h and -V option descriptions
  - Improve clarity of -s/--noupdate description
  - Add missing article in -d/--scan description

No functional changes. All technical content remains accurate.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Ahern <dsahern@kernel.org>