node-fetch: update version to 2.6

v2.6 is the current (and presumably final) minor release of v2.
Apparently these types already included at least one v2.6 feature
(`agent` as a function). So, updating the version as suggested at-rule
DefinitelyTyped#55533 (comment)

types/node-fetch/index.d.ts

@@ -1,4 +1,4 @@
-// Type definitions for node-fetch 2.5
+// Type definitions for node-fetch 2.6
 // Project: https://github.com/bitinn/node-fetch
 // Definitions by: Torsten Werner <https://github.com/torstenwerner>
 //                 Niklas Lindgren <https://github.com/nikcorg>