doc: rename README.Win32.md to README.windows.md.

That matches the name of the equivalent libpcap document, and doesn't
tie it to 32 bits.  (16-bit Windows is long since dead.)

(backported from commit b820ca8)

[skip ci]

Author: guyharris

CHANGES

@@ -89,7 +89,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       Makefile.in: Fix the depend target.
       mkdep: Exit with a non-zero status if a command fails.
     Documentation:
-      Fixed errors in doc/README.Win32.md.
+      Fixed errors in doc/README.Win32.md and renamed it to README.windows.md.
 
 Friday, April 7, 2023 / The Tcpdump Group
   Summary for 4.99.4 tcpdump release

Makefile.in

@@ -359,7 +359,7 @@ EXTRA_DIST = \
 	doc/README.haiku.md \
 	doc/README.NetBSD.md \
 	doc/README.solaris.md \
-	doc/README.Win32.md \
+	doc/README.windows.md \
 	install-sh \
 	instrument-functions.c \
 	lbl/os-osf4.h \