Replace section heading in docstring with bold text

The section headings resulted in warnings:

    None:8: CRITICAL: Unexpected section title.

    Comparison methods
    ==================
    None:36: CRITICAL: Unexpected section title.

    Distance maps
    =============

They rendered fine in the Development API page¹ but did not render on
the CLI page².

This seems like a bug with sphinx-argparse³. For now, use bold text to
avoid the warnings and allow the text to render on the CLI page.

¹ https://docs.nextstrain.org/projects/augur/en/stable/api/developer/augur.distance.html
² https://docs.nextstrain.org/projects/augur/en/stable/usage/cli/distance.html
³ sphinx-doc/sphinx-argparse#31

augur/distance.py

@@ -4,8 +4,7 @@
 which sequences to compare) and a distance map (to determine the weight of a
 mismatch between any two sequences).
 
-Comparison methods
-==================
+**Comparison methods**
 
 Comparison methods include:
 
@@ -32,8 +31,7 @@
 parameters allow users to specify a fixed time interval for pairwise
 calculations, limiting the computationally complexity of the comparisons.
 
-Distance maps
-=============
+**Distance maps**
 
 Distance maps are defined in JSON format with two required top-level keys.
 The `default` key specifies the numeric (floating point) value to assign to all mismatches by default.