docs(ref): Explicitly call out the case change for DEP_LINKS_NAME

Author: epage

src/doc/src/reference/build-scripts.md

@@ -445,7 +445,7 @@ The metadata is passed to the build scripts of **dependent** packages. For
 example, if the package `foo` depends on `bar`, which links `baz`, then if 
 `bar` generates `key=value` as part of its build script metadata, then the
 build script of `foo` will have the environment variables `DEP_BAZ_KEY=value`
-(note that the value of the `links` key is used).
+(note that the value of the `links` key is used and the case change for `key`).
 See the ["Using another `sys` crate"][using-another-sys] for an example of 
 how this can be used.