Correct list of files in src/backend/lib/README

binaryheap.c and stringinfo.c have been moved to src/common/ by
respectively 5af0263afd and 26aaf97b68, and the README patched here
still mentioned these two files as available in src/backend/lib/.

Author: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CAJ7c6TPg-=tC+fzq0tGTtmL7r79-aWeCmpwAyQiGu0N+sKGj8Q@mail.gmail.com

src/backend/lib/README

@@ -1,8 +1,6 @@
 This directory contains a general purpose data structures, for use anywhere
 in the backend:
 
-binaryheap.c - a binary heap
-
 bipartite_match.c - Hopcroft-Karp maximum cardinality algorithm for bipartite graphs
 
 bloomfilter.c - probabilistic, space-efficient set membership testing
@@ -21,8 +19,6 @@ pairingheap.c - a pairing heap
 
 rbtree.c - a red-black tree
 
-stringinfo.c - an extensible string type
-
 
 Aside from the inherent characteristics of the data structures, there are a
 few practical differences between the binary heap and the pairing heap. The