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>