[diff.cpp17.library] Fix list of new headers compared to C++17.

Author: zygoloid

source/compatibility.tex

@@ -1980,7 +1980,10 @@
 \rationale New functionality.
 \effect
 The following \Cpp{} headers are new:
+\tcode{<bit>},
 \tcode{<compare>},
+\tcode{<concepts>},
+\tcode{<contract>},
 \tcode{<span>},
 \tcode{<syncstream>}, and
 \tcode{<version>}.