DOC: another linebreak in the flow chart

one more thing i forgot to mention in the previous commit -- using
"#" in the node name for "# mutations / Mb" caused the node label
to vanish, presumably because of
mermaid-js/mermaid#5419. the previous commit
fixed this (changing "# mutations" to "Number of mutations"), but this
made the node label a bit too wide (not wide enough to trigger the
other bug documented in the previous commit, but still wide enough to
look a bit off). so anyway this commit adds another <br/> to make the
nodes all have roughly the same size.

README.md

@@ -39,7 +39,7 @@ flowchart LR
     4 --> E
     5 -..->|"(Optional)"| E
     E --> 6(("FDR estimates<br/>(TSV)")):::art
-    E --> 7(("Number of mutations / Mb<br/>(TSV)")):::art
+    E --> 7(("Number of<br/>mutations / Mb<br/>(TSV)")):::art
     7 --> Z{"Plotting FDR curves<br/>(see tutorial)"}:::ext
     6 --> Z
     6 --> F[strainFlye fdr fix]:::pgm