Fix tiny typo in htools.rst

Backtracking through git blame, it turns out than commit 49148d15
changed "\fB20,102400,16384,4\fR" into "**B20,102400,16384,4**". But
of course that's a mistake, since the markup was \fB, not just \f.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>

man/htools.rst

@@ -233,7 +233,7 @@ support all options. Some common options are:
   - the cpu core count for the nodes
   - the spindle count for the nodes
 
-  An example description would be **preferred,B20,100G,16g,4,2**
+  An example description would be **preferred,20,100G,16g,4,2**
   describing a 20-node cluster where each node has 100GB of disk space,
   16GiB of memory, 4 CPU cores and 2 disk spindles. Note that all nodes
   must have the same specs currently.