Usage 1.2, Environment variables: missing conjunction?

[Federico Bruni]

http://lilypond.org/doc/v2.13/Documentation/usage/command_002dline-usage#environment-variables
 

Maybe in **this sentence** something is missing?

"LILYPOND_GC_YIELD

    With this variable the memory footprint and performance can be
adjusted. **It is a percentage tunes memory management behavior**. With
higher values, the program uses more memory, with smaller values, it
uses more CPU time. The default value is 70.
"

I mean: 'tunes' should be a verb, right?
"The percentage tunes memory management...", if I've understood
correctly.

So it could be: "It is a percentage *that* tunes memory management
behaviour".
I don't know if this is good english, but I can understand it better.