Introduce "join" and "meet" as terms

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

content/blog/02_spa_agda_combining_lattices.md

@@ -449,10 +449,6 @@ The same is true for maps, under certain conditions.
 The finite-height property is crucial to lattice-based static program analysis;
 we'll talk about it in more detail in the next post of this series.
 
-{{< todo >}}
-I started using 'join' but haven't introduced it before.
-{{< /todo >}}
-
 ### Appendix: Proof of Uniqueness of Keys
 
 I will provide sketches of the proofs here, and omit the implementations