docs: use correct units in free.1

The free manpage used the correct unit names (e.g. membibyte) but the
incorrect unit (e.g. M ) for the human-readable option.

References:
 https://bugs.debian.org/898774

Signed-off-by: Craig Small <csmall@enc.com.au>

NEWS

@@ -1,3 +1,7 @@
+procps-ng-NEXT
+----------------
+  * docs: Use correct symbols for -h option in free.1      Debian #898774
+
 procps-ng-3.3.15
 ----------------
   * library: Increment to 8:0:1

free.1

@@ -2,7 +2,7 @@
 .\"  This page Copyright (C) 1993 Matt Welsh, mdw@sunsite.unc.edu.
 .\"  Long options where added at April 15th, 2011.
 .\"  Freely distributable under the terms of the GPL
-.TH FREE 1 "2016-06-03" "procps-ng" "User Commands"
+.TH FREE 1 "2018-05-31" "procps-ng" "User Commands"
 .SH NAME
 free \- Display amount of free and used memory in the system
 .SH SYNOPSIS
@@ -85,11 +85,11 @@ display the units of print out.  Following units are used.
 .sp
 .nf
   B = bytes
-  K = kibibyte
-  M = mebibyte
-  G = gibibyte
-  T = tebibyte
-  P = pebibyte
+  Ki = kibibyte
+  Mi = mebibyte
+  Gi = gibibyte
+  Ti = tebibyte
+  Pi = pebibyte
 .fi
 .sp
 If unit is missing, and you have exbibyte of RAM or swap, the number is in