Fix doc-string of 'fit-window-to-buffer' (Bug#36848)

* lisp/window.el (fit-window-to-buffer): Fix doc-string.

Suggested by Drew Adams <drew.adams@oracle.com>
1 job for emacs-26 in 25 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #2788
test

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