[Histograms] minor change in README.md

The README.md referred to pretty-print.py (which looks like a typo)
and pretty_print.py. This CL changes them both to use
underscores (e.g. pretty_print.py).

(doc change)

Bug: None
Change-Id: If13ccf79bfd805cd8cfa3ea3111d318bbe45b545
Reviewed-on: https://chromium-review.googlesource.com/849699Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527082}

tools/metrics/histograms/README.md

@@ -81,7 +81,7 @@ To add a new entry:
 
 1. Edit [enums.xml](./enums.xml), adding the feature to the `LoginCustomFlags`
    enum section, with any unique value (just make one up, although whatever it
-   is needs to appear in sorted order; `pretty-print.py` will do this for you).
+   is needs to appear in sorted order; `pretty_print.py` will do this for you).
 2. Build `unit_tests`, then run `unit_tests
    --gtest_filter='AboutFlagsHistogramTest.*'` to compute the correct value.
 3. Update the entry in [enums.xml](./enums.xml) with the correct value, and move