[libfuzzer] Make requested changes to docs

Bug: 901836
Change-Id: I086972b916d5b46d497a4b57ae60f64627d25551
Reviewed-on: https://chromium-review.googlesource.com/c/1321569Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605881}

testing/libfuzzer/reproducing_on_windows.md

@@ -12,7 +12,8 @@ powershell.
 1. Download the test case from ClusterFuzz (if you are CCed on an issue filed by
    ClusterFuzz, a link to it is next to "Reproducer testcase" in the
    bug description). For the rest of this walkthrough, we call the path of this
-   file: `$TESTCASE_PATH`.
+   file: `$TESTCASE_PATH`. We will call the fuzz target you want to reproduce a
+   crash on: `$FUZZER_NAME`.
 
 2. Generate gn build configuration for libFuzzer: