README: range-end is not a flag and should not be used

[spzala]

As README declares in the beginning, flags can have dash and it should be
replaced with underscore while using. Also flags or commands are emphasized
in general. The range-end is not a flag or valid doc word and should not be
used as such.

Fixes #9075

Contributing guidelines

Please read our contribution workflow before submitting a pull request.

[xiang90]

lgtm.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@fdec12c). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #9076   +/-   ##
=========================================
  Coverage          ?   76.05%           
=========================================
  Files             ?      359           
  Lines             ?    29952           
  Branches          ?        0           
=========================================
  Hits              ?    22781           
  Misses            ?     5590           
  Partials          ?     1581

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fdec12c...f6f4589. Read the comment docs.