Fix file name typo

postgres/assumed-radius-of-the-earth.md

@@ -0,0 +1,18 @@
+# Assumed Radius Of The Earth
+
+Using the
+[`earthdistance`](https://www.postgresql.org/docs/8.3/static/earthdistance.html)
+module, we can get the assumed radius of the earth (in meters).
+
+```sql
+> create extension cube;
+CREATE EXTENSION
+
+> create extension earthdistance;
+CREATE EXTENSION
+
+> select earth();
+  earth
+---------
+ 6378168
+```