doc: limit lines to 80 cols in internal README

We generally stick to 80 columns even in markdown files.

PR-URL: https://github.com/nodejs/node/pull/12358
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

lib/internal/readme.md

@@ -1,4 +1,6 @@
 # Internal Modules
 
-The modules in `lib/internal` are intended for internal use in Node.js core only, and are not accessible with `require()` from user modules.
-These are subject to change at **any** time. Reliance on these modules outside of core is **not supported** in any way.
+The modules in `lib/internal` are intended for internal use in Node.js core
+only, and are not accessible with `require()` from user modules. These are
+subject to change at **any** time. Reliance on these modules outside of core
+is **not supported** in any way.