Update properties/P000240.md

Co-authored-by: Geoffrey Sangston <geoffreysangston@gmail.com>

Author: felixpernegger

properties/P000240.md

@@ -20,7 +20,7 @@ where $\sim$ is the equivalence relation generated by $(j,x) \sim f_j(x)$ for al
 The set $J$ is allowed to be empty, in which case $X_{n+1}=X_n$.
 The $0$-skeleton $X_0$ has the discrete topology; if it is empty, $X$ itself is empty.
 
-Here $D^n$ is the closed unit disk in $\mathbb R^n$ with its "boundary" $\partial D^n=S^{n-1}$ being the unit sphere in $\mathbb R^n$.  In particular, $D^0=\{0\}$ and $\partial D^0=S^{-1}=\emptyset$ by convention.
+Here $D^n$ is the closed unit disk in $\mathbb R^n$ and $\partial D^n=S^{n-1}$ is the unit sphere in $\mathbb R^n$. We set $\partial D^0=S^{-1}=\emptyset$ by convention.
 
 *Note*: A *CW-structure* on a topological space $X$ is a filtration $X_{-1}\subseteq X_0\subseteq X_1 \subseteq\dots$ satisfying the conditions above.
 Strictly speaking, a *CW complex* is a space $X$ together with a compatible CW-structure.