fix typo

src/principal_value.jl

@@ -6,7 +6,7 @@ particular set of numbers is better conditioned (e.g. `MRP`) or obeys a particul
 non-negative rotation). In order to preserve differentiability it is necessary to allow rotation representations to
 travel slightly away from the nominal domain; this is critical for applications such as optimization or dynamics.
 
-This function takes a rotation type (e.g. `UnitQuaternion, `RotXY`) and outputs a new rotation of the same type that corresponds
+This function takes a rotation type (e.g. `UnitQuaternion`, `RotXY`) and outputs a new rotation of the same type that corresponds
 to the same `RotMatrix`, but that obeys certain conventions or is better conditioned. The outputs of the function have
 the following properties: