fix dev install instructions references

There are three name variants for the same file: README-DEV, DEVELOPERS and INSTALL.

This unifies to README-DEV. There are no other references to this name in the docs.

The change from "these instructions" to "this instruction" is grammatically dubious I believe but clarified that the sentence does NOT mean INSTALL.Unix and INSTALL.Windows that are mentioned in the previous paragraph.

After all, the sentence after is directed at people who are not expected to have any prior exposure to context.