README: the close() sentence split in two (closes #231)

The sentence in "Opening a library" had a relative clause with its "that" left out, so "returns resolves" read as two verbs in a row. It is now two sentences; the behaviour described is unchanged and matches close() in src/library/index.ts.

Model: fable-5-1

README.md

@@ -1079,10 +1079,10 @@ cached, so the next fetch asks for it again. The reasons are kept in memory
 only.
 
 `lib.close()` stops the background timer. It can be called more than once. A
-refresh in flight is left to finish, with its write to the cache. The promise
-`close()` returns resolves once that refresh, the machine-learning fetch and the
-background downloads already running have all finished. The cache directory can
-then be removed. After `close()`, `fresh()`, `freshCollections()`,
+refresh in flight is left to finish, with its write to the cache. `close()`
+returns a promise. It resolves once that refresh, the machine-learning fetch and
+the background downloads already running have all finished. The cache directory
+can then be removed. After `close()`, `fresh()`, `freshCollections()`,
 `freshFiles()` and `lib.backup()` reject with `the library is closed`.
 
 ### Default reads and fresh reads