Terminology instead of Notation as section header

Notation implies that we are going to be given symbols which
represents values or such (which arguably could be true since the
path is a particular value, and from programming we are used to
multi-letter symbols perhaps also with whitespace, but it's a
stretch), but Terminology is more accurate, as this section
describes fits the definition better: it is a vocabulary of
technical terms.

Hat tip: @bska

cmake/OPM-CMake.md

@@ -10,7 +10,7 @@ sets up declarative lists of prerequisites and content and rely on convention
 and pre-made modules to do the build. Its goal is to replace but be
 compatible with the old autotools-based system.
 
-## Notation
+## Terminology
 
 In the build system to following abstract locations are referred to: