Correction glacier documentation

Changed the sentence:

> The definition of glacier boundaries and initial volume is defined in three parameters. 

to:

> The definition of glacier boundaries and initial volume is defined by two parameters.

docs/src/model_docs/shared_concepts.md

@@ -53,7 +53,7 @@ snow modelling is not optional. Glacier modelling is very close to snow modellin
 considers two main processes: glacier build-up from snow turning into firn/ice (using the
 HBV-light model) and glacier melt (using a temperature degree-day model).
 
-The definition of glacier boundaries and initial volume is defined in three parameters.
+The definition of glacier boundaries and initial volume is defined by two parameters.
 `glacierfrac` is a parameter that gives the fraction of each grid cell covered by a glacier
 as a number between zero and one. `glacierstore` is a state parameter that gives the amount
 of water (in mm w.e.) within the glaciers at each gridcell. Because the glacier store