Note that header files should be surrounded by angle brackets (<>).

man7/man-pages.7

@@ -383,6 +383,12 @@ are always in italics (e.g.,
 .IR <stdio.h> ),
 except in the SYNOPSIS section, where included files are in bold (e.g.,
 .BR "#include <stdio.h>" ).
+When referring to a standard include file under
+.IR /usr/include ,
+specify the header file surrounded by angle brackets,
+in the usual C way (e.g.,
+.IR <stdio.h> ),
+
 .PP
 Special macros, which are usually in upper case, are in bold (e.g.,
 .BR MAXINT ).