static/frontend: increase percentage of mask-image start

The existing percentage of where the mask-image linear-gradient starts
currently overlaps some links when text-spacing is applied and therefore
decreases the color-contrast ratio of the link making the text less
accessible. This change increases the percentage and thus moves the
gradient to be closer to the end of the container and less likely to
overlap text and links.

Before screenshot:
https://screenshot.googleplex.com/By9T6ZKwv6wxPue)

After screenshot:
https://screenshot.googleplex.com/Ad4J3683xC9sQdH)

Fixes b/365624081

Change-Id: If39365e978a7747e3410a49de3a6cbbf06e9dc22
GitHub-Last-Rev: f20d142
GitHub-Pull-Request: #97
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/621839
Reviewed-by: Denis Bowen (xWF) <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Bypass: Jonathan Amsterdam <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

static/frontend/unit/main/_readme.css

@@ -31,8 +31,8 @@
 
 .UnitReadme-content {
   /* stylelint-disable-next-line property-no-vendor-prefix */
-  -webkit-mask-image: linear-gradient(to bottom, black 75%, transparent 100%);
-  mask-image: linear-gradient(to bottom, black 75%, transparent 100%);
+  -webkit-mask-image: linear-gradient(to bottom, black 95%, transparent 100%);
+  mask-image: linear-gradient(to bottom, black 95%, transparent 100%);
   max-height: 20rem;
   overflow: hidden;
   position: relative;