False-positive `proprietary-license` finding in Guava source code

### Description

Scanning https://github.com/google/guava/blob/v31.0.1/guava/src/com/google/common/graph/StandardValueGraph.java#L36 results in a false-positive license finding of `proprietary-license`, although no licenses declaration is present at all. (Thanks to @PatteSI for finding this.)

The matched text just says

changes to the graph (if the graph is mutable) but may not be modified by the user.

I guess the "modified by the user" words are what triggers the finding. However, what's a bit disturbing is that the license score is 100.0 for this match... so ScanCode is ultimatively confident that this is a license match, and we can't get rid of it by adjusting the `--license-score`.

### How To Reproduce

scancode --license --json-pp - StandardValueGraph.java

### System configuration

Ubuntu Linux 18.04