Help: Fix CMP0037 docs.

INTERFACE libraries may not use double-colons, but IMPORTED
targets may.

Help/policy/CMP0037.rst

@@ -11,7 +11,7 @@ diagnostics expect target names to match a restricted pattern.
 
 Target names may contain upper and lower case letters, numbers, the underscore
 character (_), dot(.), plus(+) and minus(-).  As a special case, ALIAS
-targets and INTERFACE library targets may contain two consequtive colons.
+targets and IMPORTED targets may contain two consequtive colons.
 
 Target names reserved by one or more CMake generators are not allowed.
 Among others these include "all", "help" and "test".