Remove extra colon prefix from inline link

Author: jmikola

source/reference/aggregation/substr.txt

@@ -13,6 +13,6 @@ $substr
 
    .. note::
 
-      ::expression:`$substr` is not encoding aware and if used
+      :expression:`$substr` is not encoding aware and if used
       improperly may produce a result string containing an invalid utf-8
       character sequence.