content/static/css: add word-break to filenames in documentation

In the "Package Files" section, filenames longer than the
column-limit overflow onto the next column.
See https://i.imgur.com/5SrSWaZ.png.

This change fixes this behavior by adding a word-break to
the files list.
See https://i.imgur.com/rw8anB9.png.

Fixes golang/go#40889.

Change-Id: I716066644436576063779f33e56e15f7eb2f3f4f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/249122
Reviewed-by: Julie Qiu <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>

Author: smasher164

content/static/css/stylesheet.css

@@ -1265,6 +1265,7 @@ code {
 .Documentation-filesList {
   column-count: 3;
   column-width: 12.5rem;
+  word-break: break-all;
 }
 
 .Versions-list {