Fix this.resolve return type in docs

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

The return value of this.resolve will contain the module metadata denoted with meta and not custom.

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#fix-this-resolve-docs

or load it into the REPL:
https://rollupjs.org/repl/?circleci=13258

[codecov]

Codecov Report

Merging #3825 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3825   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files         184      184           
  Lines        6502     6502           
  Branches     1883     1883           
=======================================
  Hits         6311     6311           
  Misses        101      101           
  Partials       90       90           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 41fd968...434c517. Read the comment docs.