fmemopen.3: Consistency fix: use "stream" as name for "FILE *" argument

Inspired by Carlos O'Donell's patches.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/fmemopen.3

@@ -93,14 +93,18 @@ bytes to the buffer result in an error.
 (By default, such errors will be visible only when the
 .I stdio
 buffer is flushed.
-Disabling buffering with
-.I setbuf(fp,\ NULL)
-may be useful to detect errors at the time of an output operation.
+Disabling buffering with the following call
+may be useful to detect errors at the time of an output operation:
+
+    setbuf(stdream, NULL);
+
 Alternatively, the caller can explicitly set
 .I buf
 as the stdio stream buffer, at the same time informing stdio
-of the buffer's size, using
-.IR "setbuffer(fp, buf, size)" .)
+of the buffer's size, using:
+
+    setbuffer(stream, buf, size);
+
 .\" See http://sourceware.org/bugzilla/show_bug.cgi?id=1995
 .\" and
 .\" http://sources.redhat.com/ml/libc-alpha/2006-04/msg00064.html